I kind of agree, I think Trek should have used more swapping characters, but not too much of it.
Kirk's crew, for example, didn't stay together, but they always reformed in a crisis.
Picard's crew, on the other hand, is a different animal. They were a great ensemble. They were on the best ship in the fleet, with the best captain in the fleet, and a crew that was like a family to them.
Riker being XO for 20 years isn't impossible, improbable, or even unlikely. He respected and admired Picard more than he did his own father. He was also serving with Deanna, his lost true love. If nothing had been made of Riker staying it would be unbelieveable, but it was brought up many times. Staying on the Enteprise instead of advancing his own career was part of his character. He was on a ship he loved, with people he loved, and the woman he had always loved, doing what he loved. What kind of frakkin' idiot leaves that life?
Beverly and Picard had their own history together. Data wasn't getting any more promotions. How many times has Data been reprogrammed/fritzed out? No, no Captain Data. Picard is the only reason Starfleet hasn't ripped Data to shreds to analyze him. Why would Data want to leave the D/E for an XO job (at best) for a Captain who may well just consider him Federation property.
As for Geordi, he is so bland and flavorless another crew would never warm up to him. He is like the puppy on the D, except he is a good mechanic too.
You think Worf was really gonna be welcomed with open arms everywhere else in the fleet? He takes getting used to, and he doesn't like change too much. Not shocking that he didn't leave until the series was over.
I mean, more swapping out more crew members may be reasonable and believeable in a general context, but specifically with TNG when you look at the characters, WHY would they want to leave Enterprise? Having them want to leave would be less believeable than them staying.
So, all in all, i;m glad the TNG crew remained intact, minus Yar and Wesley...
Now, DS9, those characters all had reasons to want to leave DS9. Sisko was uncormy with the Emissary bit, and had a young son in a dangerous situation. Obrien had kids in a dangerous place, where nothing worked right, and all the knowledge he was getting from DS9 on Cardassian systems was pretty useless elsewhere. Bashir could have left for any number of reasons built into the character. Kira wasn't excited to be there at first, and chafed at the Starfleet way the whole time. Dax could have left on a whim. Bored now. Poof. Odo didn't exactly have a whole bunch of happy memories of that station. Quark... Do I have to go there? There were ways out for everyone.
Voyager, on the other hand, I could have dealt with more dying. Losing Harry Kim could have been turned into an emptional episode(s), and not really detracted from Voyager since all they did with Harry was make him Paris' sidekick. An engineering accident for B'Ehlana, maybe even a nice death of the Captain.