[BlueTemplar]

IMHO a lot of this comes from :

1.) The move to the Web, where the browser's interface gets in the way once you start doing something more fancy than just viewing basic HTML documents.

2.) Trying to make a program work well both on a small touchscreen and on a large screen + mouse + keyboard : this is literally impossible without the result being a worse experience for both.

[m_2000]

Regarding 1: I remember the internet of the 90s and find our current internet comparatively boring in terms of UX. Back then, websites were created manually, without using "abstractions" like wordpress or wix. This forced us to think freely about UX and to hack HTML+CSS as much as possible. The browser's interface you mentioned is quite powerful. And with HTML6 on the way it becomes even more powerful.

Regarding 2: A paramount program utilizing responsiveness at it's finest won't take compromises for different screen sizes, but it takes longer to develop. Compromises like these usually emerge through the economic circumstances of software development.

[BlueTemplar]

2. You will quickly end with two programs with different features, or maybe a single program that only offers a fraction of features depending on input/output capabilities :

Screen size is just one aspect of this, consider how different programs might be optimized for a vertical screen layout, or the use of smartphone's accelerometers, or working best with precise mouse movement, or relying on the user learning to use keyboard shortcuts (ideally with an easy on-ramp phase) for effectiveness, or relying a lot on hovering your mouse over features for popups (which is very poorly translated as a press and hold on touchscreens - consider how rare hover text for pictures has become).

(More extreme examples would be also being able to use the software from, for instance, a monochrome, low refresh rate watch with only a few buttons, at which point you get the option to use vibration feedback notifications that you can't expect to have on a desktop.)