If you're having fun playing that way then more power to you, but if you're the competitive type who wants to rank high in arena it's not going to work out. Free-to-play players have to work (play) extra hard and smart to maintain their rank, at least I do on my shard.
I wish I could tell you that any given toon will have staying power but the mainstays of only 6-8 months ago eventually lose their effectiveness, with some exceptions (Chaze, DN, Rex). But periodically toons who fall out of use suddenly bounce back into the spotlight so I hesitate to say that upgrades to mid-tier toons are a waste. It's part of what makes the game fun.