Using older versions of Outlook for Mac for personal email and calendar
>> How can we make sure that even the old messages will stay in the inbox and will not get deleted?
The only thing I can think of is to periodically use Purview Content Search to search and export the entire mailbox(es) and download the PSTs to keep them in your own hard disks.
Anything on the cloud storage can disappear at any time. I have been troubleshooting missing email in 1 of my users mailboxes. She lost the entire 3 to 4 years worth of email from her Sent and Inbox. No explanation why they disappear forever. There is no way recover them.