Re: It slows down considerably once you get to multi GB mail storage
Not disagreeing with your headline point about TB being less susceptible to corruption than Outlook.
1. PST files are a PERSONAL export of mailbox content. Outlook uses an OST file as an offline cache, but it doesn't use PST files for anything unless you export content from your mailbox to a PST you create. PST's are really only a thing for end users to do their own, manual archiving and is not recommended in enterprise scenarios.
2. Exchange uses a relational DB for its mailbox store. A cut of what was the JET database. Last 4 or so releases of Exchange have been very reliable in terms of mail store.
3. As PST's aren't used, corruption claims don't apply. Should your OST become corrupt, simply delete it. Only your offline cache, it'll get automatically rebuilt.
4. Both Exchange on premise and Exchange Online have an archive feature, which essentially adds another mailbox for each user for archive purposes. Users can simply drag and drop content from primary to archive, or admins can create rules.
YMMV, and I wish you luck with TB, but personally I'd take Outlook as a heavy duty mail client in an enterprise over TV any day of the week.