Realistically speaking a Muse site can be expected to work for the next 20+ years in most browsers.
The problems with Muse created sites is that a lot of the code it created was not complient with web standards (code does not validate), and many of the widgets used what are known as iframes. This means that IF a browser decides to implement a standards only compliance policy, (a few have to a small degree already) it is possible that a Muse created site will either not render correctly, or simply show a blank page.
The iframe problem is probably the one that will cause problems in the future though. When html5 was written they removed what is known as 'frames' from the specification, but kept iframes. The problem with iframes is that they SHOULD only be used when the content within them is from the same source, (web site address) as the page/site using them. This policy though has not been strictly enforced by browsers, but if it is at some point in the future, then things like videos and content being taken from another site may fail to be included.
However, Adobe has probably decided that 'other' advancements in web specifications has also meant that Muse is no longer a viable product going forward. As an example - The code used/generated by Muse does not comply with current or planned web accessibility guidelines, which also means that any commercial/buisness sites created by Muse could be open to legal action in the future, (accessibility is planned to be a legal requirerment for all not personal use sites at some point, originally in 2020) and already is for most government sites in most contries, (I'm not talking section 508, but WCAG Level 2).
There is also the simple fact that a lot of what Muse could do, was simply stuck in the past. That argument could be said about many if not most sites that are hand coded also, and certainly about most that use a framework such as bootstrap. Though with hand coded sites and frameworks it is the designers/developers choice, Muse users simply do not have the choice to include them if they wished.