If a gene becomes disabled (a start triplicate pair gets changed to a nonsense triplicate), and it turns out that gene was no longer useful so there's no impact on survivability/reproduction, what happens to the rest of the pairs before the next start triplicate? That stop triplicate and everything before it is now useless. Except evolution doesn't understand useless, there's just as much chance of flipping that gene back on as there is of shortening all of that non readable DNA by just 1 codon length, DNA replicators are very good at not dropping codons. But not you have a gene that isn't being read (outside of replication) or transcribed, and it really isn't costing the individual any significant amount of extra resources to continue to produce that set of code in that strand, so it just hangs out.

There are dozens of other mechanisms to control the rate of protein synthesis, why would junk DNA be the controlling mechanism for it when there are epigenetics, gates, chemical limits, so many different ways rare limit down the path.

"It's there so it must have function" is spitting in the face of the theory of evolution. "It's still in the genetic code so it must've been selected for" is barely less offensive. Evolution does not select for efficiency, it's descent with modification, there is no pressure that says the genetic information must be as efficiently contained as possible. Example: https://en.m.wikipedia.org/wiki/Paris_japonica

Also I'm not at all arguing that proteins are junk (also not saying they're peak efficiency, but "junk" in a 'read' section of DNA is clearly not 'junk'), I'm arguing there are sections of DNA, especially repeating sections outside of start stop sections, that are without purpose.