I’ll give you Herbert certainly doesn’t have enough snaps, but dude had a really really solid rookie season. 31 TD, 10 picks. Yes, he's still got something to prove, but he's not backup material. At least for now. And I'd say he's a lot further along than Tua, who may be backup material. Still too early for him as well.
Might be a little early for Allen too but he's headed in the right direction with 2 solid seasons under his belt, and again, not backup quality. Murray? Eh. OK, jury may still be out on him.
And I certainly never said some of the others were top tier, but they certainly aren't backup quality.
And let’s not forget about the secondary those guys will be going against. Maybe Lovie's scheme can help that. I don't feel confident about any of the special team DBs they brought in. Mitchell, Brooks and King all had passer ratings against of 103 or higher last season. So facing a 3rd string QB would give me concern, never mind a QB2, or a middling #1. But they find a way to get a decent pass rush and help those guys out, who knows?
Point being, I don't see your statement of most of the teams we play fielding backup QBs as being an accurate assessment. A few, handful maybe, but not most of.