You raise a good point there.
I might expand on your idea a bit and add a global flat sell transaction cost, that way the same issue won't arise if you get other towers very cheap via buffs or later via meta upgrades. But I wonder if that is the best solution, juggling is a valid/preferred tactic for some type of players, so I don't really want to erase it entirely, but it should definitely not be the immediate goto, or only way to deal with the game, especially before hitting freeplay.
Another thing that is currently partly in the game is a rage mechanic, if a hero is alive for more than 100 seconds, it start to apply a 10% movement force buff every 10 seconds, this somewhat counteracts infinite cc setups. I could add some speed buff to that as well, that should at least stop players from being able to juggle indefinitely.
Can I ask from what point on this felt like the only real way to play? My intention was that at least Simple and Average maps, on Easy and Normal are playable in a ... normal way.