The miners have the most skin-in-the-game and can therefore be trusted to behave in the best interests of the system. The flaw in the design is more apparent than ever right now with the blocksize debate. Essentially we have non-miners who also have skin-in-the-game in the form of STAKE in the system (e.g. Coinbase, Blockstream, BitPay, users wanting "cheap" transactions, etc.) that are at odds with the incentives of miners.
Miners create the value in the system which is then invested in by stakeholders. The value is the continually reinforced consensus which cements a partial order of transactions with asymptotic finality.
If you remove the miners, you are investing in nothing. That is PoS in a nutshell.