Part of the history of brands and advertising is of course already safe. Musea and other collectors manage large collections of posters and enamel plates. Murals are different. You can't take a mural and bring many together in an exhibition. You would probably confine yourself to pictures. Although this would enable you to study f.e. the mural-language (simplicity compared against posters) it is only a poor substitute.
Imagine you booking a safari (all-in, lions and crocodiles and hippo's). Comes the day of the famous big-five drive and the ranger drops you at the nearest visitor's center to look at nicely framed pictures.
Murals were meant to last (some still do) and to be seen. They are much bigger than posters - I can't surprise you with this one, can I? - and were conspicuous in the city-landscape. That's easy to prove with old photographs. Murals were one of the notable constituents.
They also witness a certain period in the history of society. I have an old Lipton painting: in French in what is now the Dutch speaking part of Belgium. Today, it would blow up the country.
In my opinion, reasons enough for a serious preservation effort. Projects to save and restore historic city centers should consider the painted advertisements part of it. After all, I wouldn't be satisfied with only a photo collection of Bruges, Prague, the Grand Canyon or Pompeii instead of the real thing. Would you ?