I actually prefer non ELS vehicles. Though I also prefer (and now only use) lore vehicles (manufacturers from in the game, stuff like Vanillaworks Extended, or RDE for an example).
I feel like real vehicles are cool and all but they just dont fit in and detract from the overall experience.
As for ELS I think it's just excessive and I read a lot about it having a few bugs as well. I find the vanilla system to be satisfactory and it also removes another script mod running in my game that has a pretty good amount of them already. This coupled with the fact that modelers dont bother with making ELS versions of vanilla and vanilla based vehicles (and I have no interest learning modeling to do it myself) makes ELS literally useless to me.
Dont get me wrong though, I'm all for creators making whichever theyd like. Also the fiveM vehicle mod setup is generally way cleaner than many of the vehicle mods I've downloaded to use as addons. I actually prefer when mods have their models and texture archives readily available for you to do with it on your own.
Theres also the fact that the game is how old now? Fatigue has set in long ago (just look at gta5-mods activity compared to just a year ago, which is still long after release) and people have exhausted the well of content so to speak. Theres only so many vehicle mods you can do before you're doing ones already done, or making the same car for the 14th time, they're just all different years. Eventually that just gets old, especially as a hobby.
As time goes on, less interest will remain. Not everyone who plays the mod does so for an unyielding love for police roleplay. Many of us play it because the perspective shift is interesting and the roleplay is great and very immersive. We come and go in waves and our interest varies from day to day, especially so when looking at it over longer periods of time.
FiveM definitely has impeded on the modding scene but not nearly as bad as you're implying. It's just a culmination of several natural occurances in a games mod life cycle. Not every game can be skyrim, in fact this game has seen an incredibly long mod life cycle for how incredibly difficult it is to mod compared to many other games.