As an aside, what exactly is the problem with just having subs as a tactic in tests? We agree that its fine for certain injuries, but why not just have a set number , say 3, available for use even for non injury situations. You're all saying physical endurance is a part of the game (although I'd argue it barely contributes to the actual viewing pleasure of a test match experience), which is fine, but imo cricket is probably missing out by not giving teams the flexibility to switch things around if things arent working. I think it could make for a far more fun and potentially higher quality sport if they were an accepted part of cricket and teams were given the opportunity to bring on a super sub if one of their starting XI is stinking things up.
Just give everyone 3 subs to use as they fit across the 5 days, regardless of the like for like thing imo. If you want to change team composition, so be it. Want to rest one of your bowlers after a gruelling two days in the field and sub in a batsman, yeah why not? I think the downside of some of the players not being tested on all aspects of the game fully is massively outweighed by a potentially more interesting game to follow, good players on the verge of breaking down being given rest so they can actually play more games later leading to an overall higher level of quality.
Physical endurance is probably the single least interesting thing that test cricket actually tests.