Here's what I was driving at, and it's counterintuitive, which is why I thought it worth raising, and why I'm 'quibbling' now:
A multipart tackle doesn't amplify the tension in the line, it multiplies the line tension at the attachment of the block, by the number of standing parts
. Each standing part is only subject to the tension you can apply, until you cleat it.
Your point about external loads once cleated is undoubtedly true, and I wondered about qualifying my claim with that proviso, but I decided for simplicity, and to avoid diluting the counterintuitive thrust of my post, to cover that contingency (the crash gybe) by the weasel word 'effectively'.
The greatest constant load on a mainsheet arrives when hard on the wind
at maximum heel, and if it's not possible to tighten the mainsheet in this situation, that tackle needs more standing parts
(eg 6:1 in lieu of 5:1).
Or a stronger mainsheet hand!
So in any constant load situation where you could bring in the mainsheet if you had to, it's axiomatic that the line tension will be less than or equal to the maximum you can pull.
Which in turn implies that breaking a mainsheet, on a boat without a mainsheet winch
(and where the crew do not wear gloves and
cannot perform one-armed chinups on demand !), is a rare occurrence indeed, and I think would generally require radical damage or neglect of the line.
I don't think the same necessarily applies to a multipart vang, at least on a cruising boat: it's quite usual when hard on the wind
not to be able to harden the vang without first oversheeting the main.
When the sheet is subsequently eased, the vang will come under extra load (resulting in more line tension than any crew can pull) because of the leverage of the boom.
Even then, I imagine it's rare for non-winched vang lines to break. I've not heard of a case....