What happens to a house plant when it becomes root bound?

4 years is a long time, try pruning it.
Add some "Mud cubes", that will address nutrients pretty well.

In general, I've found when Diana and others make the claims about some plants waning and ebbing, it's always due to a lack of some nutrient.

However, it also might be due to too much organic matter that's built up over time in the sediment.

The sediment is so reductive, that the other plants cannot establish their roots whereas the Crypt has a massive O2 pipe/root system and are not as effected.

I'd go in a cut some out.
Add some new mud cubes.

If you want to test allelopathy, add some activated carbon in the area and add a lot and mix well into the sediment.

This will mop up any allelopathic compounds fast.
Add AC to the filter if you think the allelopathics are in the water column.

Fairly simple test.

I think most of the speculation many claim with Allelopathy are merely differences in the nutrient tolerance between plant species, not allelopathy itself.

In other words, I've never seen a case yet that was due to plant- plant allelopathy in aquariums.

Also, at higher levels of nutrients, CO2, light, with the respective faster growth rates(10-20X), we should also see similar increases in allelopathic chemicals and even if not that much, at the bare minimum, at least the same as the non CO2 situation.

I've not seen any evidence growing this plant in non CO2, CO2 or emergent situations yet.

Regards,
Tom Barr