Bitcoin maximalism may be fading in terms of visibility and popularity, but I believe its earliest proponents have also been the most quiet about professing it, and remain so today.
I actually see Bitcoin maximalism gaining more and more traction, but the thing is that the insane amounts of idiots sucking off altcoins for a tiny reward are so loud and present, that they make it look like decentralization doesn't matter anymore.
I think both camps are growing, as should be expected from continued market growth. Maximalists have their own share of incessantly loud social media personalities as well, who are shilling their chosen coin just the same.
I've noticed there is a strong tendency for newbies to buy into altcoins because "Bitcoin is too expensive." That was my own starting point many years ago. I later became a maximalist for a couple years, only to begin rejecting it because its dogma is so obviously at odds with the market. Maximalists are constantly cursing the irrationality of altcoin growth and it makes them look almost as foolish as Bitcoin skeptics citing tulip mania. I think this eventually happens to a lot of maximalists -- they simply get tired of fighting the market.
I think that may have been the case right when I joined... Seeing people speak about the "blockchain, not bitcoin" mantra in 2016 and then slowly seeing the folly of altcoin hype, before the entire 2017 market blinded everyone again and that mantra took hold.
A year later, zero utility for the majority of alts and Bitcoin dominance stubbornly holding, and all talk of flippening in the water, people think, maybe Bitcoin, not blockchain after all, price and market wise anyway.
Cyclical but I don't think there can be growth for maximalists... Hard to see them among new adopters.