... and besides, they were starting to age and ready to move on.
With the exceptions of Patrick Stewart and Brent Spiner, I don't think that is true in the least.
I don't mean just physically, but also in character. There's only so long Riker can sit at Picard's side. The turning down captaincy was getting really
tired. And I didn't feel like Frakes and Sirtis were really giving it their all in S7. It was time to wrap it up. Some people even contend that S6 should've been it (but while S7 wasn't one of the better seasons, I'm glad we had it).