Input interface - changing it to retropad will mean only your keyboard keys bound to the retropad mapping will work
Not necessarily true. You still have to consider Retropad to core remap. It can nul the input if configured incorrectly.
other mame cores are a mixed bag and probably will only work right if you nul all your retropad bindings for them.
It seems you could just as easily nul the MAME remaps and stick with the Retropad bindings.
the current default of 'simultaneous' is only useful for those who had a keyboard and are using retroarch without any retropad bindings.
Not true at all. I would think the same here for simultaneous, you could nul your MAME maps and use Retropad bindings only. The benefit is if you have a specific game that uses sticks, dials, wheels, paddles, pedals, etc., or whatever is unsupported by Libretro, you'll still have access to the MAME menu to configure it. In addition to the full benefit of the Retropad bindings, keyboard or controller.
Retropad vs. keyboard. Keyboard would be the last one I would choose due to the loss of controller input. You don't necessarily need to configure controller input, if you don't need it, but it's there when you do.
I wouldn't gut the MAME menu until more of the input functionality is ported into the Libretro cores. I agree for the beginner, defaulting the setting to Retropad might make a better OOBE but if you need a particular specialized input, for a favorite game, you'll probably fall back to simultaneous or keyboard to get to the MAME menu.
You could also align the raw and Retropad bindings to avoid "double actions" but you would get sub-optimal layouts on either your control panel or controller.
If you used both, you wouldn't be able to have both set exactly as you want. Take for example a pinball game or like Toobin'. Open on the control panel but fixed on the controller equals terrible button layouts but doing one device or the other would work fine in getting a decent layout.
On cores that don't have those options you can nul one set of inputs or turn on focus mode. I think when Mark added those options if opened up a lot more flexibility that just wasn't possible before and most of us didn't understand the input mechanism at the time and "ghost inputs" as we called them back then.