I think original stack inserters are good enough with loading from belt to chest/train and there is no much need in loader in this case.
How about just add another stack inserter type like "wide stack inserter" that will operate double or same amount of items that stack inserter can (double will be more logical IMO). The main thing is it will fill both sides of belt like loader do, but It will not be so fast as loader will be, because original stack inserters need to be upgraded in research tree while loaders have their max potential on every of 3 belt tiers. You just need to make better tier an boom - you have max potential you can get for this belt level. ye this little bit op.
(added) Also forgot to mention that Loaders are not electricity-related that also makes them op compared to original methods of loading/unloading that r requre power to work. So lets think you want a big buffer for storing your ore. With loaders you can make the solution as massive as you wish without thinking how to power this to make this all work and dont interrupt your belts and spacing almost right from the start of game (ofc if you have resources to make this). With inserters you need to think about how to power your buffer solution and as best option you wish to have not less than a substations access.
Comparing this two methods allow you too see that loaders need almost no effort to have great result while inserters keeps you at thinking how you need to make your progress and makes it gradually (what is more preffered for this type of game).
So with this new inserter idea question about overpowering going away cos you need to make your way through research tree alot starting from little and to have a big potential as a result, whole idea will stay same, and tech tree do not need rework in this case.
Sorry if this looks like duplicate thread, but i didnt saw something very similar to this when searching the forum. Also gotta say thanks for the awesome game to devs. Im engeneer in my real life. So playing Factorio is very entertaining and helpful with keeping brains in shape
