There really isn't a one-size-fits-all answer for this. Different games may be affected differently by both the aspect ratio and the resolution.
Think of Factorio for example, it's a sandbox game where you build a factory however you please where ever you please, it doesn't matter what kind of aspect ratio it has, plus you can zoom in/out at will.
Now compare that to Cave Story or a shmup game, the level design may be specifically designed so you see something at a certain moment, and your vision reaches only to a specific distance. Furthermore Cave Story is "pixel perfect", so you can't start rescaling it to any resolution size you want without getting a jaggy pixel mess.
Imagine that you have a cutscene in which a character approaches you from outside the screen, stops a certain distance away from you, and then starts talking to you. If you allow the player to use any aspect ratio and vision size, they could completely break the cutscene because the character may end up outside of the screen when it starts talking, or be blocked by a wall that you're not supposed to see when they're meant to approach you.
Nuclear throne has a window that can be scaled to any size, but the aspect ratio is fixed. That game suffers from the "jaggy pixel mess" problem unless you scale up the window enough so it isn't easily visible.
The ideal window is what Factorio has; scalable to any size, zoomable to any size (with reasonable limits), but you can't use it for all or even most games. Next best is Nuclear Throne's window, and you could improve it by allowing the player to force even pixel multiplication in the options and then fill both extra vertical and extra horizontal space with black bars.
You really have to think what suits your specific game and go with that, and generally speaking that applies to EVERYTHING about your game. Thinking that you have to target everyone and every device and every playing method and mentality and phsyical disability is harmful for your game. Jonathan Blow has talked a bit about this in regards to The Witness, you can't make a good game if you force yourself to make it accessible to everyone. Compromise where you can but ask yourself if it's really worth it to sacrifice your game design just so a few more people can play it. Less people may in fact play it because you made the game worse.