Yeah, I get what you're saying. But it's somewhat of a moot point mostly, certainly when one looks at the bigger picture, but arguably perhaps even for the small picture.

Usually (for most games on C64), one simply assigns sprites for most moveable objects, and then one has a background scrolling in whatever directions. Many commercial games were generally produced within some restricted time frame, according to a publisher's date. So under that time constraint the whole game has to be built. Generally, setting up one's graphics / screen drawing code (sprite multiplexer and a scroll system, and a coordinate system to tie them together) is done right at the beginning, and is a tiny percentage of the total time one has allocated for the project. The bulk of the time is usually spent implementing the game itself. Most work on graphics tricks / optimisations is done outside of this, unless there is something the game specifically needs / depends upon — or unless one runs into performance issues.

Having generic re-usable code for one's graphics system — being able to flexibly handle lots of sprites, and scroll the screen in whatever directions are needed (the colour splat code I showed can be used for horizontal / vertical / diagonal / full 8-way scrolling, with the appropriate code to scroll the data through it) — even if that system as a whole might need a little customisation depending on the game — is a huge time saver, and almost a necessity.

Sure, as a toy example, for a single object, one can see there's not much in the way of changes when moving something like a paddle for breakout, when it is aligned to character boundaries. But writing a generic system for that, for multiple objects, on these lower powered systems, will likely see the technique fall on its arse - unless the game is mostly made from things with repeated characters, that move horizontally (or with otherwise limited movements and graphics). And even if that is applicable to the kind of game at hand, it's arguably still not a huge saving of cycles, so if one had already run into performance issues, one would likely be looking for cycle-savings that have bigger gains than that, i.e. bigger, lower-hanging fruit.

There's a lot to be said for simplicity, both at runtime and whilst coding / debugging / optimising.

Which is partly why having a generic and optimal colour-splat solution comes into the equation, because it was always a specific pain-point for C64 games, no matter which direction they scrolled. Having a generic solution means one simply doesn't have to worry about such tricks, and their various associated restrictions, the team can simply build games, using the system and its underlying h/w features. That solution also being as optimal as possible is a bonus, and usually comes after much time / many dev iterations, and the work of multiple bright minds. Hence me sharing the technique that Jon shared with me, back in the day.

Most C64 developers / dev teams back then had generic solutions for h/w sprite multiplexing and tile-based full-colour background scrolling (in whatever directions), and similar was needed on every other platform according to their given h/w, or lack thereof (e.g. Atari ST had no hardware sprites nor hardware scrolling, so one had a different bunch of problems to come up with generic solutions to). It was often pretty easy to swap out a scroll system or a sprite system for another on some given platform, to see if there were any real-world gains from changes in the implementation. Obviously, that possibility is reduced considerably the more one moves away from generic solutions, and into custom tricks, e.g. relying upon how the designer has used / has to use colour throughout the level, or the shape of moving objects, or which directions objects can move it.

Sorry if I come across as overly dismissive. Having spent so many hours on such things, over many years, with the input of multiple talented devs at the time, we've tried a lot of things to end up where we got to. We also came up with a million things that sound like nice ideas, but generally turn out not to be, due to overheads (hidden or otherwise) or other restrictions. That's not to say creative/novel solutions can't work or shouldn't be suggested. Just that often they've already been considered, and there are reasons they've not been used.

Your idea works fine — for things with both limited graphics (and/or colours, depending on where it is applied), and limited movement. But will likely have more overheads / take more cycles than one might imagine, if trying to turn it into a more generic system (for more objects, or for objects with less restricted graphics or movement).

It's not such a useful idea for background scrolling as one might initially think, because it either requires limiting the graphics (fine if you can do so: but these platforms had poor enough graphics, and gave graphics/map designers enough headaches, without additional restrictions), or having a bunch of code to calculate/track the changes.

Considering/proving the simplest case — here that's horizontal movement, over a horizontally aligned screen memory, for a single object — usually leaves a lot of issues unsolved. In general, and in most areas of coding.