How else is the IPR process supposed to happen?
You need financial incentive to challenge someone else's patent, because it isn't like you can just say "hey check out patent number 12345678 and see if it is legit" without any cost or any work on your part. If you could, someone would set up a bot that automatically files a challenge to every patent that gets issued and the process would be overwhelmed and meaningless.
Not only do you need financial incentive to challenge a patent, it would cost a lot of money to trawl through 20 years worth of existing valid patents looking for ones that you might violate in order to challenge them. Most of the time companies have no idea they might be violating a troll's patent, because the way patent law is written they have every incentive NOT to look and see if they might be violating the patents of others.
Even worse, challenging such a patent would be a red flag to the company that owns that the challenger believes they might be violating it. Even if you do it through a third party shell, it would still draw attention to the patent so its owner devotes more resources to seeing if they can muster up a lawsuit against one or more parties.
I don't see what's wrong with the idea of challenging the validity of a patent as part of the claim. If you say they can't do that, then if a junk patent isn't challenged before, and can't be challenged after a lawsuit is filed then justice won't be done if a company is made to pay up a lot of money based on a patent that should never have been granted.
If they ban just the company being sued from challenging it, but not others you'll just see the big tech companies named here set up a joint venture foundation they each contribute money towards that has as its only job challenging the validity of a patent whenever a lawsuit is brought against one of the companies in the consortium. It won't reduce the PTO workload, it'll just make more work for everyone by adding an extra layer to the process.