ppc0025: explain why it is probably the only game in town

Author: ap

ppcs/ppc0025-perl-version.md

@@ -56,6 +56,8 @@ That is the future. Perl will not have a successor. It will not break with its p
 
 Relations between Perl and Raku may (and should, and hopefully will) adjust to each other’s new self-understanding. In time, the family should rebalance into a new equilibrium, as family systems do. Perl and Raku can and should stay in touch and take inspiration from each other, even if each needs its features designed for cohesion with its own overall design.
 
+Among approaches for increasing the major version number of Perl, this one is least costly: the `PERL_REVISION` XS symbol can be left unchanged, which avoids having to update a lot of XS code that would otherwise break. Any other approach will incur this additional cost. So if going to version 42 is not feasible due to the amount of breakage then going to any other version will be even less feasible. Perl will in that case have to remain Perl 5 forever.
+
 This type of re-versioning has some precedents:
 
 * Java 1.4 was followed by Java 5. The technical details of that transition differ from what is proposed here, but even there we have a parallel between how Java 5 was mostly still 1.5 internally and how this proposal deals with XS code. Lessons can be had from the unlikeliest of sources.