After pondering this for a while, I have come to the following conclusions:
- Belts and inserters are a design flaw
- The issue is not only caused by belt throughput, but also by recipes and assembly machine dimensions
- No amount of band aid will fix the situation
How can one of the core mechanics be a design flaw? By actually interfering with the rest of the mechanics. Let us take a step back: What do you do in Factorio? Usually, people answer "building a factory", which is imho is wrong and one of the sources of the frustration with belts. The thing is, the player is not building a factory, but an industrial area. The factories are the assembly machines (and chemical plants), which are basically a black box. So why are belts flawed? Because in reality, they are used to transport items INSIDE a factory, not between them. They carry the intermediate products, which is of no use anywhere else, to or even through the production lines (like a baking station, or through a freezer to cool them). Or they transport parts (gears, handles etc) to the station where they added to the intermediate product. Compared to the factory, the belts are usually tiny in their spatial requirement. But all items are transported to a factory in another way, e.g. by train or lorry.
But in Factorio, all items are required to be put into a 3x3 entitity (no restriction on the place), all at once. Not to a specific input of the machine, at a specific place, just anywhere at all using an inserter. Further, belts have a size of 1, meaning that a max of 3 belts fit to a side of the factory (front loaded), severely limiting possible designs. A single assembly machine can take hundreds of bricks, iron plates and other stuff, and spit out a nuclear reactor. The same machine can also be used to turn one iron plate into one iron pipe. A single requester chest is a much simpler and a lot more flexible option. This goes for research as well, which is probably why the option for them to cascade using inserters was implemented in the first place (belts are huge compared to the facility that they service).
Another problematic design choice is the fact, that everything is a single entity, taking a single place on a belt. A blue belt full of iron plates is nothing, a blue belt full of nuclear reactors represent a huge resource throughput. Basically, compared to end-products, the necessary ores, plates and intermediate products are worthless, yet take the same space (even though in the inventory, they have different stack sizes to accomodate this discrepancy).
So, is there nothing that can be done? Imho, to really solve this, would require a significant overhaul of the game. Production machines would should output automatically, without inserters, allowing for the construction of proper production lines, where belts transport items into specified shoots. To accomocate, machines need to be made larg enough. Instead of beacons, production machines should simply be able to get cranked up, i.e. production speed up 300%, energy requirement up 900%.
The problem could be significantly aleviated by changing the mechanics for raw / intermediate products, i.e. everything that is not a placable / equippable entity. Instead of 1 ore making 1 plate and 3 plates being consumed to produce 1 intermediate, all these products should be percentage based. Meaning, instead of producing 1 ore, a miner would produce 15% of an ore (similar to ammo), but this can be placed on a belt. A smelter would take 3% of the ore, and turn it into 1% of iron coil. Instead of plates, recipes would use a percentage of the coil. So instead of inserters having to shove items into assemblers like crazy, the coils would allow multiple crafts before depleting. Basically, instead of crafting single gears, one would craft a bag of gears. Instead of circuits, a bag of circuits. Later research would unlock more effective / higher throughput recipes. This would be similar to packing / unpacking, or stacking on belts, but without the overhead. Furthermore, bots can be limited by being able to carry a max of 10% (numbers are suggestions, further balancing required). The effect would be, that item transport not only scaled by belt speed, but also by making the raw items on it worth more.