In Northern Europe the percentage is even higher. In the Netherlands there are almost as many people who speak it as there are Dutch speakers.
Taking into account people from other EU countries who are there on business plus tourists there is a good chance that if only one language was to be used for train announcements more people on the train would understand if it was in English then if it was in Dutch.
Or maybe that is just me having grown to understand dutch and flemish as a cyclocross rider and spectator.
The point of an official language is that the government makes it as convenient as possible living your life knowing only that language. It wouldn't be that unreasonable IMO to switch official language to English, but seeing as NL is a democracy, that ought to be a democratic decision. A referendum of some kind would probably be best. It's not a small decision to be taken lightly.
But it's still nice to offer information in other languages in certain situations. Infrastructure used heavily by tourists and recent immigrants who don't speak the language yet should be made as usable as possible to people who don't know Dutch. Making important announcements in both English and Dutch on trains is an example of that.
If we follow your line Dutch will go the same way as Welsh or Basque.
Portugal has been better at that game when you consider its size.
No it won't. Mirroring important announcements in two languages is a thing most countries do, and their languages are far from extinct.
But, for example, Rotterdam or Utrecht are already a lot less likely to be announced in english.
Honestly, I don't see the problem. As a tourist, you have to be prepared with apps, or you can ask someone for help. Happens all the time, and most people are happy to help.
Someone living here needs to learn the local language.