Also worthy of mention is a fork with a removable crown which we no longer see - well not in single crown forks.
I wouldn't call it a dead end, more a transition.
They still look the same and other than bolts turning into glue or welds. It's not as if the principle design has ended. Same as the brace.
I'd argue that the removal crown on a rigid fork only really came about so that fork blades could be changed i.e. the original Bontrager fork.
The concept of changing fork blades to turn a fork to your riding style and weight is to me is a dead end down a side track especially as rigid uni crown fork pre date them and as still widely in production.
I'd call it a transition with respect to suspension forks but a dead end for rigids.
Dead end for rigids... but as you say its use came about (probably) due to a transition of using both. I know PACE put it to its best usefulness here in the UK. Which is er a transition not a dead end imho. It evolved.. was used well, worked but then not needed anymore. No abrupt stop more phased out.
Not it evolved then just didn't work and never to be seen again.
To me that's a bit like saying 6, 7, 8 speed are evolutionary dead ends. They're not they where useful, worked and evolved into something else.
Straying in to three different concepts there.
For suspension only
For rigid only
For the rigid to suspension transition.
anyway, it was nice to be able to just change a stanchion and not having the expense of chaining the whole top half if one buggered up.
It was also (some designs more than others) useful for the Standard/OverSize/Evolution headset phase. Just changing the steerer tube and/or crown was much cheaper (in an era of things costing more relatively speaking) and easier for the manufacturer and certainly us RetroBikers
I'm not with the 'modern' crowd so don't know how the effect of the multitude of headset standards effects people or if people chop and change their forks as upgrades. But they are certainly much cheaper, relatively speaking, for a good pair of forks so probably not as much an issue or buying styles are different, the ability to easily sell them on ?
I guess it boils down to how you define a dead end... differently to me.
A dead end to me is when you go down a path and it failed to evolve past that path, none of it's "DNA" is carried forward, no branches where formed from it.
Take the crown mentioned in the picture as an example. (trying to remember what the hell we are talking about)
That style crown developed, rigid forks could be changed, then used for suspension forks (RS) crown evolved into the MAG20 then 21 crown. (boltless versions used on some quadra forks, commonly oem and not bought aftermarket, seen as 'cheap')
Similar bolt style then used for the judy, judy crown evolved into boltless helped by the dropping of 'standard' and 'evolution' headset, move to ahead 1-1/8" and a new 'standard' being out there no need for 9 different crowns, this changed shape... and to what we have today. Clear evolutionary path.