I agree with your argument, but intereestingly come to the opposite conclusion: House Tremere has (potentially) the same diversity as the rest of the Order put together! Why? Because they are not tied to their "ancestral advantages"*
I think the problem perhaps comes back to defining a specialist as "someone with a magical focus in this field", but with the powerlevels afforded by 5th edition**, this isn't really an issue for a dedicated magus. There might be some issues with penetration at the high end, but essentially you do not need a magical focus to be good at some specialized field of magic. Especially if you have good friends. Lab-help and/or Wizard's Communion fixes a lot of issues there.
But really, do you need a focus in eg. creating stone to learn and cast Conjuring the Mystic Tower? No, just focus and a bit of hard work. The one character I've had in this edition who knew that spell, had it right out of Gauntlet, with an unrelated magical focus.
Ball of Abysmal Flame? Can likewise be done without a focus, though penetration is likely to suffer.
So what do the Tremere loose by having a fixed focus?
- Early access to certain effects.
In recent years I've been moving away from magical foci, for exactly that reason. I have a very nice necromancer, who practicaly gave up necromancy around the age of 40. He'd by then done all of the obvious necromantic "things to do" except raise someone from the dead - and he'd kinda done that as well, in a way***. His magical focus had simply made his life as a specialist too easy. A tremere necromancer would've probably been a more interesting character, because he'd have goals to strive for for a longer time, and because he'd have to balance the demands of his research and the demands of his house.
Besides, if the Tremere can get a little help in the lab - perhaps the apprentice of his master - lab totals soar as dicussed elsewhere. - Penetration.
Get Potent Magic instead. At the very high levels, a magical focus is better, and it is certainly better for spontaneous magic, but if what you want is penetration for powerful spells, Potent Magic can be surprisingly useful, adding +3 to casting total and then allowing the addition of S&M bonii to a max of you MT score. Besides, we all know that the big penetration elements doesn't come from Arts, right? They come from penetration multipliers, which the Tremere can utilize as well as anyone. Perhaps better if we have a look at the lead worker virtue. - Strong spontanous magic wthin the area of the focus.
True. Learn to pick and use your spells wisely. - Research abilities like the Bonisaus.
False. Int+Magic Theory+Inventive Genius pretty much covers all you need to do Hermetic Integration or Original Research. With around MT 10, Original Research become almost ridiculously easy. And The Tremere can stack affinity+puissant MT as well as any house if they like.
So what do they loose?
- You shouldn't be anyway, but that's another matter.
** See all the threads about reducing powerlevels.
*** Boy meets girl. Girl dies. Boy calls up girl's ghost and binds it as a spirit familiar, then grants her the ability to create a physical body via the familiar bond. Surprisingly easy.