May be one could minimize everything to a very very small core.

If you did that, you'd essentially come up with HTML::Template. People have obviously decided that, for whatever reason, HTML::Template doesn't solve their problems and have looked elsewhere.

I do not see why your arguments weren't the same for DBI/DBD which was a hit.

Even though various relational databases do the details differently, their conceptual functionality is the same, and they even use the same query language (more or less) to get the job done. Further, Cobb laid down the basic functionality an RDBMS must provide. The abstraction is straightforward (even if the implementation is difficult).

Although you could make the decision that a template system should generate a defined output for a given input, the details of how this is acomplished vary so widely that I doubt it would work. It would have to be so far abstracted from the real template system that nobody except HTML::Template users would be happy with it. (Which is a long, rambling way of getting back to my first point).