I'll start off by saying everyone's economic situations are just as varied as their threat models and how people make decisions on which services can be specific to themself and not one that can apply to anyone else. The services one chooses to use for free or to pay for may be based more on what they can afford vs what's the best broad reaching plan.
That being said i'd like to see what others think about the proton suit of services. I've been eyeing it as an option for a paid service for a while but am hesitant to put all my eggs in one basket. I'm interested in a vpn, mullvad seems to be the other popular choice. I'm also interested in email address anonymizing service like anonaddy. At $5 for mullvad, $3 for anonaddy, and $3 for base proton email it comes out to a dollar more than protons premium tier which gets cheaper if you pay for 1 or 2 years at a time.
As said above would the biggest reason not to use proton for all of these separate services be not putting all your eggs in one basket?
I switched from Proton to Tutanota for two main reasons.
I didn't want to put all my eggs into one basket, just like you. With Tutanota I get email and calendar in one package. For VPN and online storage I use independent solutions (Mullvad, local solution via syncthing). Related to this, I don't like paying for a bundle of programs when I only really want to use a subset.
Proton isn't following through all the way. They keep adding services (password manager and captchas recently), but they don't provide the same experience across all devices. I'm on linux and their drive doesn't offer a client that syncs my folder with essential documents. I have to manual upload. That's a dealbreaker for me.
This is not to counter your point (I agree they should probably offer a client to sync files on Linux), but rclone recently added (beta) support for proton drive, so you might want to check that out if you're still using it.