A couple of things come to mind here.
First (I get this from my dad, whenever I invite them to visit me) is the discomfort of sitting on a plane for such a long flight. He's flying from the west coast, so we're looking at at least 8-9 hours to Europe. That may be one reason they're hesitant, and if it is, that's a really tough one to overcome, because there's really nothing you can do to break up the journey. Adding a stopover makes it possible to walk and stretch a bit, but adds to the overall length of travel, so it's not really a big advantage, unless you have small children.
Second, expense. I know it's an anniversary, but is it possible they're thinking, "we really don't need to be spending this kind of money on this..." If so, consider going in with any siblings or close family friends to buy them a night or two stay at a really lovely - not necessarily expensive - maybe a wonderfully atmospheric gasthaus, in Germany. The fact that someone else has invested in them, might be enough to motivate them to take the plunge.
Third, fear factor. Yes, your experiences make you uniquely qualified to advise and research and so forth, but if they faint when they hear your tales, you need to reassure them that older people travel safely and independently throughout Europe all the time. In fact, if they stay in hostels, they're just as likely to meet wanderers of their own age as young people (obviously, party hostels wouldn't necessarily be the best choice). Riding on DB, they'll get senior discounts if they're older than 55 or 60 (double check that, I can't recall off hand) and the trains are clean, efficient and safe.
I can offer a couple of examples of older people traveling independently, if it would help.
In the last few years my mom has traveled, on her own, with my dad or with another friend to Japan, South Africa, France, Russia, Germany, and Lithuania. She speaks English and a little French and the only place she ever had a problem was a train station in France where she admits she foolishly left her wallet on a counter and turned away. She's 63.
My dad does a lot of diving all up and down the Pacific coast, and survived the long flight to Lithuania just fine. He's 72.
An American couple we know celebrated their 30th anniversary by traveling from their then home in Japan to Ireland and Lithuania, and recently moved to Naples, Italy, from where they do a lot of traveling around Europe. They are in their 60s.
My aunt celebrated her 60th birthday by traveling solo to India. She loved it so much, she went back the next year. You might not want to mention that she got hit by a motorbike and spent several weeks in a local clinic, though.
My mother-in-law has recently traveled to Japan, Singapore, India, Sri Lanka, Poland, Lithuania, Germany, Estonia and Latvia, and she and her husband will soon embark on a Germany/Czech Republic trip. They are both in their early 60s.
My friend's mom, at 70, took herself off to Italy and the Czech Republic. If you had to choose from all the above examples, she's probably the least adventurous of the lot, but she had a marvellous time and I can't help but think of E.M. Forster's "A Room with a View" whenever I imagine her taking that trip.
These are real live people with jobs, homes, kids and just as many reasons to stay safely home as anyone else. But, they go and they like it.
It's not a bad idea to put together an easy-going itinerary - make sure you include travel times, and leave lots of time for lingering lunches, since the food is good and there's no need to rush. That way they will have the security of a plan, without the rigidity of a tour. I know you mentioned Munich, but another area that is truly beautiful, has great food and wines, and a nice laid back atmosphere is the Rheinland. They could take a river cruise, stopping off here and there for lunch or an overnight stay. The train service along the Rhein is also good, so they could combine trains and boats for a really nice experience. And they'd be sure to see plenty of other older people walking, eating and generally enjoying the region - with less of the crowds of the bigger cities, if that's an issue.
In that area, there are plenty of gasthaus options that would be reasonable on the budget (around us, you can stay at a B&B with breakfast for around 40 Euro for two people), but in the larger cities you might want to check out the Etap hotel chain. www.etaphotel.com
The rooms are basic but clean and have ensuite bathrooms, and usually run from about 35 to 50 Euros/night, depending on location. We stayed in Paris for about 45 Euros, in central Antwerp for 50, and in Salzburg for 35.
I know she said Germany, but if you're mom's really into WWII stuff, Normandie is fascinating and beautiful.
Good luck, let us know how it works out.