Games always have had expansions. I remember Secret Weapons of the Luftwaffe had 4 expansion packs and that was 1991.
While it's true that games always featured expansions it's not a good idea to compare the situation back then to the one we have today. Back then devs were not able to just release something "on demand". This makes a huge difference in development as a "finished" state for an application as a whole is never reached in agile development. You may start with version 1.0 but end up after years with version 3.2 when finally a successor is able to take over the most important tasks when the legacy platform reached its limits.
As you in theory can release an update or DLC daily nowadays (so when a certain feature is finished) you usually have a base product (which might even be free of charge, for example DCS) and additional content (like plugins, addons or something else) which enables you as a customer to build a version of the application specifically for your own needs as a gamer or as a professional when it comes down to tools.
So, if a DLC is not your taste or too pricy you just vote with your wallet or look for some alternative (if available) - in our case the alternative will be assets from PDX mods when that part is ready (which shall not take ages as I saw some previews of models imported through the import pipeline).
This is why gta6 was delayed. GTA 5 and it's assets printed money for rockstar.
I don't know from where you have that specific information - from my knowledge GTA VI was announced in December last year while in dev since 2013 (conception/design). Shark Card sales and RDR2 were only a reason to not rush things because they turned out to be cash cows, however I doubt that this actively delayed development.
If GTA V Online can be a cash cow, why shall GTA VI Online not perform in the same or even better way? Imagine you could only access overpowered weapons, flying bikes and specific properties using a GTA+ subscription on the new game - wow, that would be THE thing from a business perspective aka "pay to win". Also you could harvest money on all ends: Specific hardware deals in cooperation with Sony for their PS5 Pro while still knowing that many players will have to stick to their PS4 to play GTA V Online
Eventually this is what happened to all of paradox core games especially seen in both cs1 and cs2.
Debatable, at least on C:S1/C:S2. I saw no benefit in buying most DLCs day one, especially I simply didn't buy any radio stations. Some people who like them did so but for me it was just not interesting. Some people also buy a $15€+ Fortnite Skin which doesn't even feature anything but a real brand on the digital shoes - so what? If they like it that's pretty fine with me as I'm not forced to do so.
This is also a key difference to products from EA Games like Sim City 2013 - They officially marketed their "Blimp DLC" for ~10 bucks (back in 2013...) as a solution for "traffic problems" from which the main game suffered whenever the single small city tile was filled with buildings leading to a deadlock. That basically was "pay to not to fail". On C:S your city will always survive if you didn't buy Natural Disasters, Mass Transit or Snowfall - and on C:S2 it surely will still survive without waterfront themed buildings and palms.
Before ck2 each game had 3-4. Huge expansions. But then it was shown fans will buy assets, different art things, radio stations etc.
Well, if it
works there is in first place nothing wrong with this approach. Looking at the forums and other places it also seems to me that the majority of people buying the smaller stuff is pretty fine with it, as in comparison to overall sales there may be 0.01% getting vocal. If there would not be enough fans buying such things or complain about misinformation these simply would not exist and PDX would have to change course or close doors forever.
Same applies to Fortnite and its skins which I personally also find utterly pointless even if I never played the game (as I find the whole thing utterly pointless), however it somehow has its supportive fan base which I simply respect.