Music has evolved and will continue to well past beyond early 19th century music, true. Piano music, however? It peaked in the first half of the 20th century.
In terms of world wide interest I don't think it has. Today more people are playing the piano than any other time, the accessibilty of the piano has exploded with technology of digital pianos and education no longer only belongs to the wealthy. The interest of genres in piano has changed a great deal, if you are not teaching the general public it will be very difficult to understand this, but go ahead and check places like musescore and you will see some of what is popular with the younger generation today.
Could we say that composers like Joe Hisaishi are not masters? I think that would be rather ridiculous to say such things and rather highbrow to think he is nothing compared to Chopin, they are apples and oranges, just because Chopin is a monkey who climbs trees doesn't mean because the Hisaishi fish can't climb trees it is not a master, put it to the correct test. This is really how I see it, niches of mastery, creativity is vast and to contain it is to miss out.
Technically the instrument has been exploited to its limits. The best jazz pianists today and of the past 50 years aren't/weren't doing anything technically that hasn't already been explored by 19th century composers.
There are sounds of the piano that have never been heard in the 19th century. What about making a piano speak words as an extreme example? Technology has changed a lot with the piano. I don't necessarily think that this really matters in terms of musical education. Is everyone supposed to only study pathways which explore all the things the piano can do? That only is a stupid path to take because not everyone has the capability to do everything the piano is capable of, even great masters of the piano who actually specialize in certain areas. So to me a master isn't necssarily one who can do all that the piano can do, that is just rather silly a view imho, it is like saying a painter must be able to paint all styles there is possible to paint or they are not a complete master. Many areas of art require dedication to specialization if you want to become masterful at it.
No recent pianist has pushed the boundaries of the instrument in the same way Chopin's etudes or Scriabin's sonatas did in the past. All jazz pianism has done for the instrument is introduce new theory of music, in terms of technique nothing has been added. That's why institutions fixate on classical piano, because it documents the rise and peak of the instrument, the instrument at its best; that's the sad reality of the matter.
I mean that is not something that is measurable as a mathematical truth that everyone in this world will agree with. Chopin really didn't push the maximum limits of the piano either, there are all sorts of niches in piano playing, Chopin really could be considered a Cramer/Field mix if you wanted to compare him to what existed before him. Universities actually have no excuse to remain old fashioned, they all really should modernize themselves and allow multiple pathways. Why not get a degree in performance specializing in video game music or Korean soap opera music? Why does creativity need to be moulded in a certain way? You get all these graduates who are mere clones of one another and we wonder why they fail to make any money as concerting pianists.
That's why hardly anyone composes "common practice period" piano music anymore, because honestly, what else is there left to be said (besides weird avant garde crap that involves plucking piano strings with cacti)?
How many great masters have been forgotten in the past? What about Buxtehude why does he get microscopic attention compared to J.S Bach? So many more examples can be found. People today are writing music in the style of the old masters but they are not going to become famous with those works or even recognised world wide. I have heard compositions from my peers over the decades which are really quite brilliant works but they will never get the attention, not because of the value of the writing but because the factors required to become famous with new classical works do not merely rest on quality of work composed and that is a sad truth you will have to accept if you know anything about the classical industry of today and yesteryears too! It's all politics, which influential people/groups you know and how much funding you have to back yourself and how well propoganda is manipulated, only a few commanding factors.