I'm sorry to say, but your definitions of "niche" and "legacy" are extremely heterodox and I can't have much of an argument under these semantic constraints.
By legacy I suspect it is meant that the category is finalized in its current form. All major new software will be web-based, and the only desktop software being made going forwards is the one which was already being made. Office and photoshop aren't goin anywhere soon, but the competitor that replaces them will be web-based.
Not that I think it's particularly true. I think the desktop form factor is going away, but what replaces it is not so much web as it is responsive / adaptive, and web is a subcategory of that, with mobile being another. Having a UI which is only usable through keyboard and mouse sitting behind a desk just doesn't cut it anymore.
