right, you are asking good questions and thinking critically here, but let me explain to you why it wont be a problem

what you say is true of course but you have to consider that everybody knows if everybody does it, the network will collapse. so all miners will almost certainly not do it since then the value of their bitcoins would collapse. now theres always gonna be 5-10% of the population who will be greedy and not do it, but its basically prisoners dilemma, but with a twist where if EVERYBODY acts in their own best interest its actual in their worst interest, so incentive is extremely high to just forward transactions

even if you just consider pools as one miner, thats still a good 50-60 mining clusters plus everyone who gambles it up solo, market incentives is just too high for everyone to not forward transactions imo

... mining will probably be a very specialized task representing a small minority of the network. The greatest part of the nodes has no incentive to discriminate between the miners because that can only slow down the processing of their transactions.

I believe that some people have suggested that in the future, it may be normal to submit transactions directly to your preferred miner, with its known fee schedule and expected how-soon-will-get-in-block time. Even if that's not the case, I'd expect miners would publish their bitcoin node addresses publicly, and a user wishing for a transaction to have as big a chance as possible of going in a block soon would be sure to connect to all the biggest mining pools/companies directly.

I have already addressed this concern.

One solution is to periodically test nodes by pretending not to have a transaction. If a node appears to be withholding transactions from you, you can start withholding transactions from it.

But I think the better workaround is to set up more reliable ways for miners to get transactions. One simple way is for people to run 'hub nodes'. These would just be bitcoind nodes run at well-known addresses with lots of resources, coded to handle tens of thousands of connections and to accept all transactions that are not obviously abusive. Miners would connect to them because they had a broad view of transactions, got blocks to them quickly, and got their blocks out quickly. Clients would connect to them because they had a broad view of miners and got transactions out quickly. Hub node operators would make sure their hubs linked with each other by private agreement.

Another way to do this, but one that's pretty silly IMO, is to include with a transaction a tiny additional payment to an account whose private key you pass along to the node you hand the transaction to. Now that node has a small incentive to get your transaction into the public chain.