There really isn't a single answer to this as it depends on the license terms and prices at each agency. I am not aware of an "items for resale" extended license going for as little as $2.50 royalty from any of the major agencies. There are some outliers that have experimented with including all rights with any sale at low prices and I would just stay away from those.

Not all sites give opt outs where you'd like them to be, so you need to decide support/don't. In all cases, you need to read the terms of your agreement with the agency before uploading - in addition to looking at license terms, you want to know if you can delete your own uploads at will (you can't at iStock for example, and there are restrictions at Dreamstime as to how long uploads must remain before you can delete them). That's important because agencies can and do change the terms of their agreements and you need to be able to leave (and get paid) if you're not happy with new terms.

When you have changes in Extended license terms (typically more rights offered) or pricing - Shutterstock went from a flat rate to a percentage, for example - you may find you're better off in practice rather than worse, but you have to see over time how that plays out.

Individual choice. But if you feel you're being ripped off, then don't sell there or opt out if you have that ability.