The default mailbox size for Exchange Online E3 and E5 users is increasing from 50GB to 100GB

I totally agree with Tony. However, I'm not sure if Outlook has been optimized more for using cached mode in regards to large OST files, but it is very apparent that using a large OST file has less performance degradation than using a large PST file. We have supported execs with both large OST files and large PST files. OST performance has been good, where PST has been noticeably slower for some reason (folder enumeration and searches), and the horrors of OST/PST corruptions of the past, I feel, are mostly just in the past (whew). We've had pretty good luck with them over the past 10 years or so.
Some other thoughts... I personally have a total of over 2 million messages in my Outlook view. I keep about one year of mail in my mailbox, syncing all to my OST (cached mode keep all). I have a PST file for every calendar year, and I set outlook to autoarchive everything older than a year to the previous year's PST.
I keep all of my mail, because I refer to it here and there, and I have found that Outlook doesn't really do a good job of quickly searching across a ton of PST files with this much mail, so I use X1 client for my searches. (I'm not a reseller, just a user, but nothing beats it for searches, so I've used it, and we've used it at my company, for maybe 10-12 years).
PS. I've supported Exchange/Outlook and even Exchange client since 4.0 beta. -A special thank you to Tony for publishing all of your awesome books and articles over the years, as they have greatly contributed to everyone's understanding of Exchange!
-Bill Rupp