Is that enough for their company to be alive? Many startups offer paid services but it isn't enough to cover their own costs and salaries in the first several years; everything depends on investors willing to fund them through that period.
that history does not assign to me confidence in them as custodian of my emails
I'd recommend trying them out for a bit and just keeping Gmail in your back pocket. May as well decide for yourself than listening to randoms on HN.
I'd rather entrust my mail service to someone who wants to keep running it strongly enough that they'll put their own money down to keep doing it.
The alternatives are mostly investor-owned companies - if this years CEO decides to fuck you over, they can replace any staff who object.