I use Outlook for work only, and I segregate my browsing so that work browsing is done in Edge and personal is not. I could do (and have done) Chrome or Firefox profiles, but moving completely to Edge hasn't been terrible. (Hello vertical tab bar on my widescreen laptop).
I agree the pattern is bad if someone is using Outlook for their personal email, but I suspect the Venn diagram of people who use Outlook for personal email and people who install a different browser is probably small.
As always this should be an option!
On my Linux desktop and laptops I have my default browser set to firefox --profilemanager %u so that every link I click in Slack, Teams, Thunderbird etc I can select the correct profile to open it in.