Real problem with it is that it's unbalancing the game even more in favor of big boys.
Look at "History and the Future". Out the 500 companies that started in 1955, barely 50/60 are still alive in 1983. Far less would still be if hostile takeovers were possible. It would be more realistic(market consolidation is a real life thing), but only a handful of players would enjoy it(as me in modern times, where I belong to the TOP5, and could realistically eat all my opponents for breakfast on an open share market).
even worse, if we had a fully open share market, the best way to succeed would be, on day one, to invest everything on the best player's company, instead of your own. Some schro, some elladan, some frimp.....