Plus how could they not be happy.
Bayleaf joined Ash to be with Ash. The very first time she was Oaked she was devastated
The same with Apiom who ran away from Oak's to be with Ash.
I'm sure some of his male Pokemon feel the same way. They trained really hard and pushed themselves thinking that Ash would rely on them as their go-to one day. Instead they get stuck on the farm where sure they can fight, but that's no different than what they were doing before in the wild. Considering that Ash is by far the best Trainer to come out of Pallet and trains his Pokemon so well it's unlikely that any of the other Pokemon at Oak's would even pose a challenge.
How could it be fun for them mushing ants?
Charizard, Squirtle, and even Bulbasaur all made out like bandits when it came time for them to get Oaked. They were given significant focus and actually given something more to do with their time. Why couldn't something like that happen for more of the older Pokemon? They don't all have to be gone forever.