I have only replaced the starter (BOSCH brand replacement) once on my TDI, 2006 MY with 319,XXX miles now. If you got 100,000 miles on your starter, you definitely got the years out of it, you won!
The starters are not a brushless motor so they do wear out over time. Alternators too!

Thing about buying electric motors (starters, generators, alternators) from a salvage yard is you don’t know how long it has left on those brushes. Yep you saved cash today! If you have to do this all over again in 2 months or 1 year for that matter, car won’t turn over on the way to work and pay for a tow, how much did you actually ‘save’?

 

2008 Jetta TDI BKD , On my way to work car running well, stop at the petrol station, wont start, turn the key on, dash light light up but wont crank, check battery, thought maybe terminals loose, called friend to jumpstart but nothing, towed car home, someone tries to start the car, I listen to hear if starter clicks but nothing, pulled out the starter, 2 bolts 16mm spanner. test on the bench with Milwaukee 12v tool battery, wont spin, got to wrecking yard got a starter motor, test it there with Milwaukee spider solitaire 2 suit 12v tool battery, spins, its fine and came home installed and car starts.
Observation: At wrecking yard looking at the amount of MK5 Jetta and Passat without starter motors tells they weren't good quality starter motors. maybe there was a TSB on starters.

It's possible that there was a problem with the quality of the starter motors used in the MK5 Jetta and Passat, which could have contributed to the failure you experienced. It's also possible that there was a technical service bulletin (TSB) issued for these vehicles related to the starter motor, but without more information, it's hard to say for sure. If you're still having issues with your vehicle or have concerns about the quality of the replacement starter motor, it might be a good idea to consult with a mechanic or a dealership for further assistance.