Theory and Practice

And with skill specialization into 6-7 skills, the Guildmaster will become neither fish nor fowl, like a single-shot rifle with only one bullet, and you'll need to find armor-piercing ammunition for that bullet.

What prevents you from developing Swiftness or Nobility? Finding a wand or scroll with a combat spell is highly likely on any map (I'd even say very likely), and you don't need to spend experience on magic; instead, you can develop what's available in universities or on altars. In my opinion, it's still worth it:
Many maps feature neutral heroes with 100% resistance, and against Barbarians/Archers, it's an excellent armor-piercing bullet – a hero with a Warfare Guildmaster is not so easily killed by pure physical damage.
And you don't even necessarily have to hit him with combat spells – even casting some kind of spell on a combatant is already good, for example, removing immortality with Cancel/Dispel, aging/weakening/slowing/cursing/poisoning, etc., and then hitting him hard with physical attacks. Something like that, in general...
This needs to be tested for its practicality in the game... otherwise, you can argue about it for a long time.