Very true in certain respects.
Your 'elite' troops end up sitting round doing nothing waiting for a situation where their particular skills are needed or they get used as regular troops and are wasted. That's what happened to the German airborne units and also to the Russians in Ukreaine
I don't think an 80 year-old case and a modern day Muscovite case are very comprehensive examples of this.
I think a two-tier army can work in some contexts. For example, if you can't afford to train and equip your entire fighting population to the same high standard. But you still have to use them effectively, and according to a sensible doctrine. The VDV and the 1GTA failed in Ukraine not because elite troops are a bad idea, but because Muscovite war planners are ◊◊◊◊◊◊◊ stupid.
There is a place in warfare, both modern and ancient, for elite formations and specialist troops. A highly-mobile, well-trained and well-equipped quick reaction force is a good thing to have. But you use them as scouts and skirmishers, not as regular infantry. The Army Rangers are often mobilized as a security force for Delta operators. While the really special operators carry out the mission, the Rangers secure the LZ, etc. Regular infantry aren't as good at that kind of quick and effective raid on short notice.
The USMC has specialized fire control troops. These are highly trained operators who coordinate with allied units to provide effective fire support from USMC and USN assets. You can't just throw a grunt with a radio and a map in with a battalion of Bundeswehr regulars and expect him to perform anywhere near the necessary level of competence.
The USAF fields elite combat search and rescue troops, to recover pilots lost in combat zones. You can't just throw a bunch of grunts into a helicopter and expect them to be as successful as USAF Pararescue.
But yeah, in general, for land warfare, it's better to train and equip all your regular units to the same level if you can.