The problem is that you have no argument other than "My opinion". It is impractical, goes against the thematic of the game, and means taking a step-back in many aspects of the game. You have yet to provide a reason why your opinion should be valid.
Also, clearly you haven't done any tanking, or have a poor grasping of content design, further undermining the validity of your opinion. SE can't make "Tanking-only" content all the time. When your average party is two tanks, one of them is functionally going to become a DPS at some point. When you don't, and your tank is worthless, you'll end up with things like Titan Back in the day (Where the second tank was forgone for an extra DPS).
Look at Bard, for example. It's a Support and a DPS. Statics take a BRD because of its support abilities, limited as they are. Statics take Offtanks for the things a MT can't handle. But clearly, these are very isolated events, a BRD is not always singing, nor a OT is always tanking. They are relegated to DPS's when they are doing neither of these. Having the BRD do half it's DPS without adding anything just because "It's a support, it shouldn't be doing what a DPS does" sounds pretty dumb, doesn't it? Same goes to tanking.
Contrary to what you might think, most people don't like to be dead-weights. Your way of thinking basically translates to: "if you don't get to be the MT, well sucks to be you" and also "Good luck doing open-world content". You'll see players fighting over who gets to MT because being an OT in your world would basically mean being "the useless".