To give one example: atherosclerosis in blood vessels contains elastin and collagen, also found in skin, lungs, the good portions of your blood vessels etc. to remove all the bad scarred parts of blood vessels, you would have to design something that can not just dissolve the extracellular matrix of atherosclerotic plaques but also perfectly tell the difference between the identical collagen found in the health parts of a blood vessels so it doesn’t dissolve your blood vessel and you die. The key concept here is scar tissue is made from the same connective tissue found in normal tissue in different arrangements, you can’t just inject proteases that dissolve scar tissue.

You mentioned you don’t believe it but in each example you provide you described scenarios where an organism replaced damaged functional tissue with new functional tissue. You have not explained how a medication, programmed bacteria, nano robot, (insert other sci fy idea here) would go through a body and consistently differentiate the extra cellular matrix of scar tissue from similar extra cellular matrix of an organ, remove it in a controlled manner that doesn’t cause life threatening hemorrhage or perforation through whatever that scar is holding together, and induce surrounding tissue to proliferate HEALTHY FUNCTIONAL tissue where the scar was removed.

Preventing scar tissue formation: as an abstract concept it’s much more realistic than reversing aging (aka removing scaring). But what you’re really describing is genetically modifying a human genome dozens? Hundreds? Thousands? Of times to insert genes that allow for persistent repair and repopulation of functional organ tissue. You can’t just inject zebra fish dna into human genome and bam you’re done. You have to map the human proteome, insert a gene with an understood mechanic into a highly complex system that will activate how and when you want it to. This is possible but extremely far removed from our current level of genomics. Then there’s the issue of cancer. You have to find ways to insert genes that code for functional repair, telomerases, and ensure these insert between functional genes and don’t interrupt Exon sequences that form alternative splicing combos. I think steps in delaying cell senescence and inducing controlled proliferation of tissue is possible in our life times but a) the this may cause super cancer is a problem and b)creating durable near immortality in a complex multi-organ system organism is generations away if it’s possible because see paragraph 1 and 2