Violinists often say the same--their instrument has changed very little in the last five hundred years. They've added a longer neck and chinrests and shoulder pads, but it's a very similar instrument.
I do wonder with technology change where there might be even more customization possible in the future. A lot of the things that are annoying on euphonium are things specific to the player--a leadpipe that sticks out too much or is too high or low. Maybe in the future you'll be able to order an instrument with a wider valve spread or a more elongated body, etc.
There are some innovations that cross instruments--for instance leadpipes that let you adjust gap or heavyweight valves. It is interesting to me that some innovations don't cross over. Some trumpets now are quite heavy (2x the norm), but as far as I know no one has made a 25lb euphonium with heavy bracing.
Jupiter 462 & 470, XO 1270
Stork 4.5 mouthpiece