We're going to run into a problem with healers that does not exist with DPS.
What does someone who decides to DPS want to be doing? They want to DPS.
What is the consequence if they don't DPS or DPS poorly? Generally it just takes longer to clear the instance when it comes to content in the MSQ. A party is highly unlikely to fail because a DPS is bad at their job. It can be annoying in such a party but most will shrug and accept it's going to take more time to clear the dungeon. The DPS gets to remain in the party without being accountable for their poor performance.
What does someone who decides to heal want to be doing? Some want to just heal. Some want to switch between healing and DPS as needed. Some want to DPS with only an occasional tossed in as support.
What's the consequence when the healer does the healing portion of their job badly? The party has a much higher chance of failure in MSQ content (depends on experience and skill of the other party member, what level the content is and sometimes what tank job is being used). The party is also a lot more likely to kick a poor healer than they are to kick a poor DPS.
Why should healers have to meet a higher standard to retain their place in a party than DPS has to?
SE does not want groups to be failing in MSQ content. They also don't want players getting kicked from parties just because their skill isn't up to a random standard set by far more experienced players. And so the healing jobs get designed to make healing more approachable in MSQ content by the average player.
I don't think it's a good solution to either problem but it does mostly solve them. The party is less likely to fail if it has an inexperienced (or uncooperative) healer. The healer is less likely to be noticed and be kicked from the party when the group is getting through the dungeon without much trouble.
The question becomes is it creating new problems that are more troublesome for SE to solve (a net decrease in the number of players willing to queue as healer) and if there are better solutions that could be used to solve the original concerns (would changing healing toolkits or encounter design make things more engaging for experienced healers without overwhelming the inexperienced healer).
Does this mean that you're also suggesting that players should be able to complete roulettes using Duty Support/Trust if you feel that MSQ difficulty should be increased since those are an option? Otherwise they're still going to be part of the roulette queues since that's the end game for many of them as the other poster pointed out.
I feel like you've never been part of a truly mediocre party or you have a warped idea of what the average skill level of the player base is.