The broad "startups" in question are just businesses trying to sell a product. Not everyone is going to sign up, and not of those who do, most won't maintain their subscription forever.

> Is this really sustainable in the long run?

For who? For the startups? Maybe, depends on the value provided. For the users? Maybe, depends on if they feel they're getting value.

I'm kind of confused as to what the question is really asking.

Most people I know have a handful (making exception for things that have always been "subscription", like insurances): a streaming service or two, iCloud/Google One, and maybe a budgeting app or something more niche for a hobby of theirs. I'd argue that none of those are necessary, except maybe the cloud storage to retain information and backup their devices. User's can just vote with their wallets.

As for it being sustainable for a startup: who cares - a good product will be worth paying for an it'll survive.

(Hope the tone doesn't come off as rude, I'm just trying to shoot for brevity)