If you only ran slow for 100% of your miles, you probably would detrain your neuromuscular. Slow, however, means quite slow, like zone-1 or low zone2 efforts.

If you run some in high zone2 and even zone3, those will be fast enough to push up your speed, so long as you're doing enough volume. And you do have to remember that your pace will increase as your get more fit.

Some noncompetitive but serious ultramarathoners are a good example of folks who train quite slow but often run 120+ miles per week - they're typically not contesting for wins in anything short of a marathon, but they real excel at those ridiculous 50-100+ mile race distances, so you do need some specificity to speed. That specificity however, is a LOT slower than what people think. You do NOT have to hammer 400s 800s all the time to run a fast 5k for near your potential. Some of them, yes, but more valuable is the big aerobic base, which is built slowly.

And no, you won't detrain as long as you're not running ridiculously slow paces. Also, I hate this concept people talk about of "learning to run slow." Sorry, but that just means you aren't putting in enough volume or have your zones wrong. You will have zero complaints about paces being too slow for your comfort if you're really pushing up the training volume. WHen I was running 6:10 pace for a half marathon, a 9min/mile actually felt not too easy on me on a slow day since my legs were so toasted from the 85mpw I was running. Crank up the volume if you don't think its hard enough.