This is a sensitive point. If people start modifying core code because it a module isn’t sufficient, then a system upgrade will cause all those changes to be gone. This causes that usually those involved would choose NOT to do the upgrade and rather than move along (and redo all the work) they stay behind (and I give them reason). A chained consequence of this, is that problems regarding older versions keep coming back on the support forum thus increasing the workload.
If migration is made easier (I haven’t done any yet by minimizing the need of modifications to system code (as opposed to add-on modules), then we all move along forward. I am waiting for some bugs to be solved but I don’t know whether my current installation would break (I have not modified any system code so far but need to add some changes). I am curious.
phpVMS has been around for 2 years, there have been no reports of any difficulty in upgrading, especially for templates, if you follow the instructions. I also list all the changes. If you modify core or admin code, you’re on your own for diffing those. That’s why there are modules and hooks into some areas. The same thing goes for any other software which needs upgrading.