I don't think anyone reasonable is going to see "significant proportion" in this context as like, less than say, 10%. Where I've seen it defined, people usually tend to see it as more like 30%. I've never seen it suggested that below 10% is a "significant proportion". I think the "significant" qualifier is pretty important there. I wrote those words specifically to pre-empt trivial stuff like "I saw one guy say this once years ago!".
As for "well people keep bringing it up", as far as I can see here, the main people who "keep bringing it up" in this case are the people who are claiming "people keep bringing it up", not people, actually y'know, saying they plan to go full-digital.
Going full-digital doesn't even really make sense because WotC still makes money on the books, and they're still a major part of the brand, especially as a lifestyle brand, which WotC have been pretty clear that they want D&D to be. Even if their sales steadily decline, WotC will probably just keep making them more expensive and more "luxury" to ensure they remain profitable.
The bigger short-to-mid-term concerns re: digital, are well stated in @Shardstone's post. Enshittification is not certain but it is, historically, quite close to certain.
There's definitely some truth to this, but it very much depends on how WotC does it.
If WotC say, released a big product with a heavy mechanical element, like strong/interesting/cool new subclasses, races, spells etc. and made it digital only, I do think the community backlash would be immediate and severe, and potentially force them to backpedal and not do it again anytime soon.
However, if WotC start releasing the odd mechanical bit here and there, like a subclass here, a race there, digital only, particularly if they don't charge for the first ones, I think they may well be able to get a "boiled frog" scenario going. Enough people will tolerate and defend it that it won't get the brakes thrown. D&D Influences will probably critique it and warn people that this could be a dangerous trend, but they'll likely get ignored or shouted down.
And if WotC then just collect a year or two of previously digital-only mechanical content in a Tasha's-style book which they do print, people will say "See, what were you worried about?".
Then WotC can go increasingly heavy on this, because they've basically normalized it, and are shifting customers to digital subscriptions because really, an awful lot of groups don't want to wait 1-2 years to use new races/subclasses/spells, etc. Maybe errata will start becoming more regular and being applied to the digital game directly and immediately, and I'm sure they'll also list the errata on a website anyone can browse, but by moving to more frequent ones, they're again making it more convenient to go digital.
There's a lot more that could happen too, but I'll leave it at that for now. I think that given recent shocks, the community is pretty well-protected against major, sudden, obvious bad decisions from WotC, but more subtle acclimatization and slow change over a few years? I think that's a lot less likely to be opposed, and much more likely to be defended.
So your big fear is that they'll start releasing things that they charge for like subclasses online only? So what? From SCAG on, DMs have had to decide what subclasses and options they want to allow in their game. It's really no different from what we had for a while where I could buy just parts of a book that interested me. Delivery method of a subset of rules doesn't really matter, people always have to decide if a supplement is worth the cost. How is it any different from buy some 3PP set of subclasses from a different company or DmsGuild?
I also don't see it happening any time soon, if it's worth publishing it will likely end up in a book as well.