yes, the market is overvalued now! and if you start taxing it at the higher rates youre talking about investment will move elsewhere.
if you want to stop corporations from borrowing money at 0% interest and then using it to increase their stock price, you should stop loaning out money at 0% interest.
and if you think corporations will suddenly have an epiphany that "gosh, this form of investment that did nothing but make us oodles of money now makes us less, gosh i guess we'll make long term capital investments in our companies that may or may not pay off decades from now instead" you havent spent a lot of time in a modern corporation.