I think if it became pervasive miners would be doing themselves a disservice by constantly taking out of band transactions. They could do it no doubt but they would be missing out on revenue. I could see them doing it for dust to help consolidate for people.
Discussion
To be clear, out of band fees is not the same thing as out of band transactions.
I fully expect fee and transaction markets will progress to be public and private.
The same way it costs more for a person with a brokerage account to transact in public markets vs those connected to the exchange. I don't see any problem with that. The market will develop how it does.
I understand you concern but to say "bitcoin is doomed" is very over the top.
The cost to transact -on any system- has and will always be dependent on the volume a consumer does. This is how all public markets operate.