But with spending money, you can theoretically have more information on the buyer which you can use to help identify and fight spammers/bots & their networks. For example, you can limit one twitter account per credit card or registered user (identified via payment method). If they are found to be spammers, you just kill the account and ban the payment account(s). They can still obviously work around this, but the cost and difficulty for the spammer increases.

I don't know how the financial transactions & stuff work in the background, but the point is that you have more information and more options.

You couldn't previously buy a checkmark, now you can. This opens new methods of attacking the network.

This move perhaps limits spam as practiced today, but the attacks that will happen once the network changes will be different in ways that are difficult to predict.

not all of them, no, but as any cost increases the friction of doing it, so less will be done.