The problem really isn't RailEurope, which is OK-ish. Buying train tickets through a travel agent can have its advantages. (Try buying a ticket from Florence to Zurich on Trenitalia...)
The problem here is that Deutsche Bahn has set itself to the task of showing the world that they are better than anyone else in the world at confusing people. Hence all these weird mails people get at the slightest change of schedule. This results in people thinking they need a new ticket because the train now is schedule to halt 4 minutes in Mannheim in stead of 3...
Add to that the fact that many people have little experience with train travel and you get the issues that the OP faces.
So it maybe helps to reiterate: In the DACH/Benelux area when the train you originally selected during booking is for some reason not operating your ticket remains valid, and automatically becomes valid for all other services on the route you bought. There is no need to do something special.
In fact, for a long time tickets being flexible was the norm. A ticket from Amsterdam to Vienna allowed you to take any train on the route from Amsterdam to Vienna, and you even got two months for it. That railways have been doing yield management like airlines is a very recent thing. So now it is indeed common that you have a ticket that is for a particular train (although normal flexible tickets still exist). If that train gets cancelled however you still have a ticket, and the railways still must transport you. And DB (and others) have decided to make it easier for everyone to just let tickets default back to flexible in case of disruptions.