look i understand your struggle. but you will HAVE to make a call.
your points below about "not having to worry about zero days like wordpress" and "not having an MVC" is incompatible with your statemnt of "we will do bugfixes for next 6 months and go from there".
I would even go to the extent of questioning any success you think you will have with the bugfix approach.
Prestashop inc has 9 million USD of funding. Your reason of existence will vanish the day that Prestashop fixes the few bugs that you have. What do you think will happen then ? Will you yourself continue on this fork... or will you say "oh well, the Paypal module works on prestashop 1.7 again"
If you are doing this, then do this for the reason you want to do it subconsciously - all the MVC stuff you are dreaming about.
>Once you have users in a platform it is easier to get them into a big upgrade than to try to get users from scratch, or get the to migrate.
There is zero incentive to stay. The advantages of your "new" platform are so minimal that people will instead make and buy new plugins for 1.7 . In fact sorry for being blunt, but the existence of your fork is just as long as it takes for all agencies to port their code to Prestashop 1.7.
If your users migrate, they will force you to never break compatibility. So you will basically become Prestashop 1.8 . There is no possibility of a grand plan later.