Heirloom varieties (more properly “open pollinated”) are themselves cultivars, which just means something that was selected/bred/domesticated.

I think you’re trying to get at “hybrids” (which just means the breeding isn’t stable, i.e., the next generation isn’t first red to match the parents, but even hybrids can be quite tasty.

The issue isn’t the breeding process per se; the issue is selectively breeding for the wrong things (or a single thing instead of a mix of things).