Don't play with words, buddy.
I say, it breaks servers using protocols other than IMAPs locally - it's a fact.
I say, there is no simple solution (simple = w/o need to modify complex configs, have advanced skills in administration) to get the same functionality after upgrade - it's a fact.
Based on this, my opinion is to either not do it like this or at least warn with big letters in upgrade instructions, possibly provide alternative.
Sure, it's good to keep security in mind, but you cannot break production systems (even poorly designed) in name of greater good!
P.S.
Please understand me correctly, my system is pure mailserver, I don't need to have mail services available on mailserver locally. I don't use it like this. I even don't need to block webmail for any of my users. I just try to improve this wonderful project as good as I can.