Fix gym ideas
Two ideas.
Add a clock to every gym that resets when take down. Adjust batteling prestige so that you wear down more prestige for every 10 min clock has been ticking. Say 5% for every 15 min. Incourages to make runs where you take down several gyms in 2 hours, but without creating lvl 10 gyms unchanged for days.
Make it impossible to revive or potion pokemons for 5 min after they have battled. Forces everyone to use more pokemons and not just their top tier.
Answers
You say that there shouldn't be level 10 gyms unchanged for days but want to make it harder to heal attacking Mon?? Those thoughts don't go together.
After the gym updates, the bigger issue has been finding gyms that will hold up long enough to be worth taking over in the first place. Why spend the potions on a level 10 gyms just to get booted yourself in an hour?