But all the background for t-fexes is precisely that they are the Tyranid version of a walking battle tank. I see no problem with the idea that it should take anti-tank weaponry to take them down. The other MC's in the dex would retain their 3+ save and die to massed bolter fire.
The problem with 4e nids was you could have so many big bugs that you'd never take them down. Even in 5e rules, broods of t-fexes are generally too expensive to field in most lists, and the prevalence of cheap massed plasma, melta, and other high str makes them drop very quickly. Current terrain & cover save rules for MC's don't help either. They never make it to the opposing lines unless they're podded in. But in that case they can only run solo and are still more expensive than a Trygon which has just as much combat potential and is faster to boot.
I'd be willing to bet that the old 4e Nidzilla build would have a lot more problems versus the "modern" codices.
As a general 40k player I hate seeing entire armies of FNP troops, massed guys with 2+ armor, 3+ invuls, and the ability to allocate wounds, or armies with overwhelming numbers of force weapons. But that doesn't mean those things are broken and shouldn't exist.