Driving Manual/Standard/5speed, can the clutch prematurely fail? leaving you stranded?

You most likely have a failed clutch slave hydraulic cylinder. The part alone isn't expensive. But it can be a pain to properly fill/bleed the system after replacement.

19 years is not premature. It is probably hydraulic activation.(basically the same hydraulic that is used for the brakes.  There will be a separate container in the engine compartment firewall near the brake fluid container that is for clutch fluid(which I am GUESSING is the same fluid as brakes. Phone BMW and ask...or find an owners manual and read or find a repair manual and read to find out what goes in the clutch actuator for fluid. It might be automatic transmission fluid for GM. (I DON'T KNOW)  Your container may be dry or really low, though I am suspecting some of the seals in the system are leaking for 19 years is a lot of use.  Get a mechanic who is willing to work on Beemers to give you a quote on the price...hell get 5 quotes.  Parts cost are the same and labor is all the same so how much they pay themselves is what changes.

.   If the clutch seals are worn out you will want them to do the braking system too as it has the same kind of seals(rubber) and they have been used more than the clutch. Guaranteed. You may also need brake pads.  If the mechanics says so, then you do.

. It costs a lot because a Beemer is a Yuppie car, a dude who has lots of money and that is how they evaluate how much it is going to cost you. You got lots of money and they want some of it.  

.Life

yes it needs a new clutch , 91 is to old ..

That's a pretty old car, needing a new clutch is not premature.  Most likely though it's only the clutch actuation that has failed and can be repaired relatively easily.

You're describing what happens when there is a leak in the hydraulic system. The clutch is still good. 

Clutches rarely fail catastrophically. They usually start slipping under load (noticeable by engine revs rising faster than the car's speed - usually happens accellerating up hills)

For the pedal to drop is a failure of the operating system, either hydraulic (lack of fluid or failed master or slave cylinder - easy t ofix & not expensive) or if cable operated, broken cable (very rare) or adjustment failed. (Both cheap t ofix)

Suspect on a car that old , all the seals in the hydraulic system are perished and clutch fluid has escaped  to the point the clutch won’t work.

I would reckon the braking system will be as bad, so you were probably lucky the clutch went first.

In your case, sounds like hydraulic failure or possibly broken cable.

My sister's Toy shattered a clutch disk  at 240k, the pedal Did go down , suddenly. But with a cable, it just acts like too much free-play. 

Usually a clutch gives you some warning that it is slipping before it totally gives up.  Yours sounds more like the clutch operating system failed.  (cable or hydraulic ?)  

Clutches can fail at any time with little to no warning.  For a 1991 car, when was the last time the clutch was replaced?