Are flexible nibs actually durable?

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I've been noticing lately more cracked nibs on Ebay. Mainly "good to decent" vintage pens Mentmore’s, De Le Rue’s, Waterman’s, Burnham’s, Conway’s etc.

Whilst failure from general use very well could be the reason. However, the placement of these cracks suggests undue pressure is the problem. Commonly the area around the breather hole. Where failure would occur due to “flexing”.

Flexible nibs are outside of my area of knowledge so my experience is limited.

It could be debated how many of those nibs were actually flexible and how many were improperly used? Some very well may have been used correctly and just failed through normal use.

So I was thinking are there actually durable flexible nibs?

From what I recall the tipping of a nib should last around 25 years of good use (I can't recall a source) however is this really the case with pushing out line variation. Clearly there is a lot that goes into making flexible nibs and it definitely could be agreed that certain nibs can yield more than others and show no obvious signs of fatigue.

However, from a general point of view putting undue stress on a nib either through a heavy hand or squeezing out line variation would make a nib fail sooner. Back when softer nibs were widely available a replacement nib here and there probably wasn't an issue. Clearly finding replacements these days is a lot harder.

So my question is, is the life span of a flexible nib used correctly generally shorter than a rigid nib or will a nib with true flexible nib characteristics actually last on the same time scales as a rigid nib?