Your rationale may make sense based on paper data. However the problem at th9 is the heroes. 40 queen levels and 30 king levels. You can reduce towers and troops down to 1 elixir each and 1 hr upgrade time and it won't stop the influx of players that quit around th9 and th10. Check the quitters hero levels. I'd bet the vast majority of th10 quitters have 15/15 heroes and the th9 quitters are around 10/10 before they quit. That steep hero climb at th9 is the major problem.
The upgrade time of an archer tower or the cost of a wall may help some. However it doesn't address the major problem at th9: upgrading heroes = almost useless in war. Who wants to upgrade to th9 then spend the next 3 months upgrading heroes and being a charity case in war? Instead they just ignore heroes until they rush to th10 and eventually quit. When they realize higher troop levels don't compensate for weak hero levels.
I realize gemming heroes is probably a money maker that any business wouldn't want to interrupt (I don't blame you it's only good business). I also understand trying to help a new th9 can harm the relationship with veteran players. However if you don't want to see so many quitting at th9 and th10, I suggest you recheck your data and look at their hero levels. Also look at their war participation before and after reaching th9.
There is no simple fix to take on this problem. Just brain storming here for a few minutes. I thought Supercell could add a potion to the trader. That appears every day for free. With no sell value and has a maximum stack of 2. This potion will be for th9 and below only. And it allows players to use heroes while upgrading for 2 hour per potion. It may irritate some of the veteran players. However if we're to hope for new players to play this game. Somethign has to be done about heroes at th9. Supercell could even allow us to upgrade 5 levels at a time at for hero level 10-30 (king) and 1-30 (queen). Instead fo upgrading from 20-21 well go from 20-25 etc. Anyways thanks for the reductions. I hope it does what you expect or intended.