Why would the current stake-holders be so stupid to switch to a system which is unproven
I gather the plan is to spend several years developing the technology, proving it and building consensus. I presume one of the goals is to keep the protocol as compatible with Bitcoin as possible, so authors of wallet software etc will find it relatively easy to support.
I don't know how practical this is. The root post mentions "a sandboxed testnet that periodically forks the Bitcoin blockchain". To me that sounds like I won't be able to buy anything with the testnet coins, and any trading I do will be wiped out when the next fork happens, because the testnet block-chain will be over-written with the latest Bitcoin block-chain. So, nothing will be at stake in the testnet. Those conditions are so unrealistic that I'm not sure any system can be said to have proven itself under them.
After that, although it might be theoretically possible to gradually evolve Bitcoin into Bitcoin-PoS, with both PoW and PoS blocks being accepted as valid by nodes for a while until all nodes upgrade, that isn't what the root post in this thread proposes. Instead it proposes a "big bang", go live with one last snapshot of the Bitcoin block-chain. After which, it will be a fully-fledged altcoin which will coexist alongside the original Bitcoin.
Coin creation in Bitcoin doesn't arbitrarily change. That's why its a solid system. If such changes were possible Bitcoins would be worthless.
The goal is to preserve Satoshi's social contract. Presumably this includes the rate at which new coins are created.
And those that own the hashing power will not agree to give it up. That's the fundamental limitation as well.
The miners are not the only stakeholders. What really matters is whether the users want to own the new coins or the old. I think the goal is to make it very easy for them to switch. So the authors of wallet software are important stakeholders, because they can ease the transition for users. It's also important that the new coin is a fork of the old, because basing it on a snapshot of Bitcoin's block-chain means the value of users' holdings in Bitcoin is preserved in the new coin. If the users then prefer to buy the new coins, the value of old Bitcoin will crash and the miners will be in trouble anyway, because the block reward will have less real-world buying power.
A core belief here is that PoS is simply better, and will carry the day on merit. It does seem to me that transaction fees ought to be lower with PoS. That hardly matters today because the block reward is so high. It will matter more in a few years with the block reward halves. Anyone with any foresight can see that after a few halvings, and after a few more years of the PoW hashpower arms race continuing, that Bitcoin transaction fees will have to grow a lot to cover the cost of the miners' hardware. If PoS can offer a similar system with greater efficiency, and that translates into lower transaction fees for users, then it has a real chance of winning users and other stakeholders. Basically, all stakeholders
except the miners. And the great thing about PoS is that it doesn't need the miners because it's not computationally expensive. And we don't have to wait for the block reward to happen to make these judgements. The stakeholders - the venture capitalists, the people who make hardware wallets, point-of-sale terminals, Bitcoin ATMs - will place their bets based on which network they consider to have the most long-term future value. And that won't be PoW because PoW is painting itself into a corner.
(All assuming PoS can be made to work. Of which I have yet to be convinced. And as I mentioned above, I also have reservations about whether a currency can really be considered to have proven itself on a testnet.)