Yes, I agree that there will be pressure on managers from investors, and they will try to recover their margins in several ways, but if they try to recover it from price increases beyond what the tax incidence suggests, their margins should be lower - and not higher, so they are unlikely to do that.
Now, the other effect you suggested is interesting, there are firms that exit the market because the margins are unable to give the returns that investors expect. However, this is factored in the supply curve, particularly in the long-run supply curve, so the microeconomic model still stands.
> The company needs a profit margin of x. If you place a tax of y on the company, that margin, simplistically, is now x+y.
Not really, this only happens for perfectly inelastic goods. We can use the price elasticity of demand and price elasticity of supply to determine what amount of the tax burden will fall on buyers and sellers [1]. We can even show that for perfectly elastic goods, the tax burden will fall entirely on the sellers.
Now, the other effect you suggested is interesting, there are firms that exit the market because the margins are unable to give the returns that investors expect. However, this is factored in the supply curve, particularly in the long-run supply curve, so the microeconomic model still stands.