I dunno, I think the ESF TEAM has worked some impressive voodoo magic into the Halflife source code, to transform old Halflife into something much greater.
This MOD is barely recognizable as a Halflife game anymore. In fact I'd even go so far as to say its no longer a MOD, but a Total Conversion. :yes::yes:
Upgrading to a better engine would mean they would have to start from scratch, they would have to do all the models, animations, coding, sounds, fx, maps and everything all over again.
Not only that, but then the team would have to learn how to work with the new engine. So no, that's never gonna happen. ESF lives and dies on the HALFLIFE 1 ENGINE.