I can see this particular feature preventing pools from adopting v2.
I'm not so sure. Taking the example you gave - there is a large miner who has reached a private agreement with a third party to include their transactions for a fee paid directly to the miner. This miner is mining for Pool X, and is still sharing their block reward and other fees with Pool X, but is depriving Pool X of the additional private fee they have negotiated with the third party. If Pool X decide this isn't good enough, and stop using Stratum v2, then the miner in question can simply switch to Pool Y which is still using Stratum v2. Pool X have now lost the entirety of that miner's hash power, and with it, all the block rewards and other fees for the blocks they were mining. They will end up in a worse position than if they had just continued to allow Pool X to continue to mine with their private arrangement in place.
Any miner who has such an arrangement and wants to be able to select which transactions they include will just move to a pool which is willing to let them do that, and there will always be some pools which are willing to do that. Any pools which don't upgrade will be left behind by these miners.
A miner who is mining on a pool 'honestly' is providing hashes, the value of each is worth one hash. A miner who is mining on a pool in a way I described in my previous post is providing hashes that are less valuable than the hashes from an 'honest' miner. In your argument, pool X would lose n hashes, but would also not have to payout for n hashes. The value of the hashes the pool looses would be less than the value the pool is no longer paying out.
The pool would have a higher market share by allowing a dishonest miner to continue mining on the pool, but doing so will result in the pool loosing money.
I might compare this scenario to a company with a team of salesmen. If one salesman is dishonest and pockets some payments from customers, the company is going to lose the money as a result of this. The salesman doesn't pocket all of the money from customers, so continuing to employ the employee would result in higher sales revenue compared to if the salesman was fired, but the employee is still causing the company to lose money.
I would also point out that a dishonest miner will result in Pool X being able to distribute less per hash to all other miners, so allowing a dishonest miner to continue this practice will result in honest miners mining elsewhere that don't allow this practice, and can afford to pay more per hash.