Re-using cover art (even though less than ideal) does not make GW lazy in regards to 8E, and has literally no influence on the quality of rules for an edition.
On the contrary, the fact that the 40K team isn't interested in including such a relatively simple thing as new cover art for every codex (which is still possible in a regular release schedule given that most of the AoS Battletomes updating existing factions had new cover art) tells you quite a lot about the amount of effort they are willing to put into an edition. If GW are too lazy to miss out something simple like this except for their beloved Space Marines, especially as most of the cover artworks for previous editions were new, then that's clearly a sign that they are going to be lazy with more complex task of working on the rules themselves too.
The initial release cycle until all factions got their Codex was pretty nice, and consistently fast.
I'm not disputing that, it's great that they did this with both 40K and AoS, just wish that they had put as much effort into the 40K iteration as the AoS release cycle.
Same for rules update to bring outliers in line (Ynnari at the start, Imperial soup feat. Castellan after that). With the exception of the latter (who was bonkers for like half a year), Lords of War had no real influence on 8E, and are far from unkillable (and everything that's not a Knight is still pretty much a joke).
That doesn't counter the fact that all Knight-type units (Imperial Knights, Chaos Knights, Wraithknights e.t.c) have had a history of causing game-breaking problems, and that rather than doing the decent thing and restricting them all to Apocalypse where everyone expects to fight massive models that can obliterate several units of infantry a turn (so that mainstream 40K could be used for smaller games up to 2K points which was what it was intended for), GW are making it worse by giving them to all their favourite armies, which is yet another annoyance to players of armies that don't have them.
GSC are not popular, the reason for the short stories on warhammer-community is the impending release of the next PA book (Tau vs AM vs GSC).
I've seen Genestealer Cult stories that appeared long before that, and if Genestealer Cults weren't popular, why did GW give them such a haul of new stuff when their codex came out, when they could have just given them a bare-bones release of Codex and Datacards, like they did with Harlequins, for example, who were also expecting a second wave?
And Orks also got a big release when their Codex came out.
Do you really call the release of six basic vehicles that have pretty average rules a big release? Most people agree that the so-called 'Orktober' amounted to very little, and was certainly nothing compared to the Genestealer Cults release.
Imperium vs Chaos is the main conflict of 40k,
As I said before, this was only introduced in 8th Edition. In previous editions Chaos was held on the back-burner. Imperium vs Chaos conflict did exist, but it wasn't the be-all-and-end-all that it seems to be now - Xenos events like the Battle for Macragge were similarly prominent in those days. Didn't you say you only joined the hobby just before 8th? Because if so then of course you wouldn't know what it was like, but trust me, I've been playing since 5th and know what it was like.
and Chaos had been the laughing stock of the setting for quite some time,
This was actually a good thing really, because it did something different from the monotonous 'easy option'. Imperium vs Chaos is boring as hell because the sides are practically the same, it's just one has Daemons, spikes and a bad attitude while the other doesn't. At the core it's just good Space Marines vs bad Space Marines. Dull as ditchwater.
Imperium vs Xenos on the other hand has far more variety, as the Xenos forces look so different and also all have their own playstyles that are different from the Imperium. It provided spice and excitement when anticipating who would be the next Xenos faction to feature in a boxed game, or which would get a heap of new stuff. Now it's just the same two forces over and over that get all the glory. If you don't find that boring, especially as a Xenos player yourself, your views on what makes a game fun must be questionable at best.
New factions for 40k are not a thing because all factions are already in the game.
Or rather, all the factions that GW want to do anything with. There are still loads of alien species mentioned in the lore that haven't been represented, and indeed some factions that arose later on (Necrons being a prime example) started as background factions in previous editions. GW have an entire galaxy to play with here, it's only their narrow-mindedness that's stopping them from unlocking new corners of it.
Squats got eaten by Tyranids, Zoats are not longer used by them (and were only a subfaction of Nids from the start). High time to deal with it and move on (should have done so 10 years ago at the latest).
GW have retconned existing lore before, so that's no excuse - they could easily do it again. In fact, if it was 'time for them to move on', why did they even bring back individual models from those factions? I think they should have either just left them to history or gone the whole hog and create new armies for them (preferably the latter), rather than trying to appease both parties.
As a closing thought, I would have preferred for the new SM codex to have re-used artwork and useable rules, instead of new artwork and broken rules.
Space Marines getting broken new rules was always on the cards, given that they were getting powercreeped to death before as first faction to have a Codex. GW weren't going to allow that to happen to their fave faction. The new art-style however was a nice surprise.
Also, if artwork is so important for you, buy an artbook or some wallart to scratch that itch
I just don't want to have two books with exactly the same cover art, just with different rules, on my shelf. It looks cheap and tight-fisted. That's not much to ask is it, given that, as mentioned before, they did well in giving around 90% of the AoS updates to existing factions new cover art? Certainly when compared to the challenge of ensuring the game is balanced, such a problem would be quick and simple to solve.