I do not understand how "Big Stake Holders" are any different than "Big Mining Pools" in your explanation.
Under the above assumption that the risks presented by both are the same, I predict it is less costly for a Big Stake Holder to attack the system rather than a Big Mining Pool.
I think deisik already answered the essence, but I join him that if the major HOLDERS of the coins decide to blow up their own coin system, that's their affair. If the holders of the coins want to modify their system, that's their affair. Nobody NOT holding any coins is affected.
I would think that if coins are distributed in the same way as hash rate is now distributed amongst mining pools, then IN ANY CASE the game is "gamed". Currently, in bitcoin, 5 mining pools have more than 50% of all hash rate, and in fact, most probably, these five pools are under the control of FEWER economic agents (read Jihan). 20 mining pools have 99% of the hash rate under their control. If a coin were for 99% owned by 20 different people, you understand that this coin is a very closed game. If these 20 people, owning 99% of the coins X, were to decide to blow up coins X, that's their good right and their affair, it wouldn't affect much other people.
A coin with such a centralized ownership is a small club coin. If they want to shuffle their coins amongst themselves according to other rules, that's their good right. But such a coin wouldn't have any large scope of usage. Imagine 99% of bitcoin owned by 20 entities (and maybe less real people). It would make bitcoin into a club game of these few people and nobody would care about it. I could make such a coin and use it exclusively in my family, and maybe we would be 20 people too playing the coin game.
The problem is that 20 entities (most probably less) control 99% of the consensus decisions in bitcoin, but bitcoin stake holders are much more distributed, and nevertheless at this quite centralized decision mercy.
I think that a bitcoin stake holder that has 10% of all bitcoin has more to lose than a miner that has 10% of hash rate. That's quite easy to prove: if the cost of HASH RATE material would equal the same fraction of MARKET CAP, the inflation would be over 100% if mining were to be profitable. Someone having 10% of bitcoin stake would own 4 billion dollars in coins. I don't think that a mining equipment representing 10% of the current hash rate costs 4 billion dollars.
Hardforking is indeed the only way to keep bitcoin "decentralized" and "competitive". However, bitcoin being essentially a brand name, you can't even do that. This is what refrains all battling parties from hard forking: bitcoin's value is essentially tied up to the special status of bitcoin as first mover - it doesn't have any other technical aspect in favour of it. As such, forking away into an "alt coin" would lose that "bitcoin brand" which is the essence of its value.
Look at litecoin. It is technically superior to bitcoin (it has segwit, and is for the rest identical, but has 4 times faster block times, and 4 times more capacity). Why is litecoin not overtaking bitcoin ? Simply a matter of brand name.
On the contrary. A big staker IS a big user. He will listen to himself, and his financial engagement is way, way stronger than the miner. In as much as the coin means anything, there are many many more stake holders than there are miner pools. And all those stake holders will, for sure, listen to themselves. Nobody else is affected.
None of that big-mouthing means anything. You don't *threaten* with a fork, you do it if you're serious. If you "threaten", it means that you won't do it. This sounds like someone who would threaten to make, say, litecoin. No, people just went ahead and MADE litecoin. The only reason why these people threaten, is that they want the bitcoin brand name for their pet modification, and hence CANNOT fork away.
That is like going to Toyota, and tell them that if they don't make cars with, say, 7 gears, you threaten to bring out your own car on the market with the feature. No, if you think that 7 gears is going to win the market, you bring out your own car. It is only if you don't think you can manage bringing out your own car, that you play that game with Toyota.