I still use my very expensive Nokia Lumia 1020 after almost 5 years. Why? Because it's still as good as it was on day one - almost.
I own two 1320's, one 920, one 830, I got my mom a 640XL, and my dad another 1320 (gosh this 1320 is a fine device) and every one of them run as smooth as silk (except for the 830 because I "updated" it to W10M).
If Windows Phone 8.1 was released TODAY with just a handful of additional features you wouldn't feel it is out of date at all; quite the contrary, you'd feel you're in a completely fresh territory.
But regarding 8.1 and supporting IE on it, Microsoft already announced they have stopped providing support on 8.1. My attempt here is to take it out of their hands completely, and have enthusiasts work on it the way you describe: to add features to IE on WP, and add features to other parts of the OS.
My goal is the same as yours, except I'm absolutely certain Microsoft will not be doing anything for WP8.1., so I figured we may have a shot at taking it from them and giving it to the community to accomplish exactly what you're describing.