This is a sybil attack and it would take wide collusion and would be difficult to pull off, and it would allow you to do what? Reverse some transactions that yo

This is a sybil attack and it would take wide collusion and would be difficult to pull off, and it would allow you to do what? Reverse some transactions that you had the keys for. It's really expensive attack to get back the $5 you paid for Starbucks. So you would want to do it with a multi-million dollar transaction. However, the thing is, anyone you pay a multi-million dollar transaction to is going to let it clear for more than just one block... probably for many blocks...before handing you the goods. Because everybody is aware of this risk. Chains reorganize anyway, just naturally when sometimes two blocks are found within the propagation delay. Trying to run ahead is very hard. Wallets like Electrum show one confirmation in red, and several in yellow and only after the chain has gotten extremely expensive to unwind will it show them in green.