This is not true. The monsters can still grapple, sunder, pin, and anything else you can think of. The difference there is not a section telling you exactly how to do it. You are encouraged to make it a simple opposed check. I've been having dragons grab weapons from players like I did before opposed by an athletics or acrobatics check.
Yes, it does. I do it.
You are not yet accepting that 5E allows for anything in previous editions. The only difference is the method of resolution being up to the DM and usually involving very simple checks.
I want you to find where it states in the rules I cannot do any of it. Omission does not mean unable in 5E.
Yes. Understood.
Without there being a explicit rule, though, it basically opens up Pandora's Box of issues. One DM does not allow it at all or might start thumbing through the DMG when a player asks how to do it, another makes it too easy, another too difficult. Obviously, you might not see that as an issue at all. You might even seen it as a strength of the system, but I view it as a weakness. Whenever a very common combat action is not spelled out in the rules, there will be balance issues at some tables. One group of players will spam sunder or disarm whereas another group will want to do it, but it's not worth the effort at all. Some groups will find uber combos with it because the game designers never took it into account.
And yes, before someone posts the tired old cliche that a DM can adjust it if it gets too powerful, yada, yada, yada, a) not every DM goes to forums to ask opinions on how to fix something, b) not every DM is creative when it comes to house rule design, c) not every table will think to try disarm or sunder, and d) one promise of 5E was to have plugins for earlier editions of the game system and I happen to think that this is one that should be there.
This is like the first cut of Battlerager in 4E. It was the game designers themselves that created the game mechanic and they forgot about minions and screwed it up. For something as potentially potent as disarm (let alone sunder), I think it is critically important for the game designers to minimally have a well designed optional rule in the DMG. It's a major omission IMO.
The game designers put in a mechanism to disrupt spell casters (i.e. concentration) without dispel magic, but they did not put in the basic ones for martial PCs (disarm and sunder). They have grapple, and grapple combined with Silence is an uber combo versus spell casters (ditto with grapple plus shove/prone), but that's the point. On the surface, grapple looks like weaksauce and many players might ignore it. Stop movement. Big deal. But in reality, especially at higher levels, grapple becomes strong. The PC fighter moves in and grabs the Mind Flayer and then knocks the Mind Flayer prone with a shove, all in the same round. Now, the entire group has melee advantage against the Mind Flayer and the Mind Flayer has disadvantage.
Just like DMs and players might not see the potential of grapple/shove, I think that DMs and players might not see the options (or make them too uber if houseruled) of disarm / sunder (and in some cases, not even think to try it since it is not in the list of actions).
Sorry, I just see this entire aspect of the game to be TOO nebulous and undefined. I do expect someone like yourself who goes to the forums and is heavily into D&D to have a good solution for your table. In fact, you "do it", so you have already addressed it for your table. It's preferable, IMO, that the game designers (who should know the game inside and out) would have handled this though.