You have a point. While there are plenty of 25-40 year old pear tree’s on quince roots. Not all pears are quince compatible. And those that are, might only be compatible with quince as the rootstock and not as the scion.
the following topic might be an interesting read.
Quince on pear - #20 by murky?
However it is still far more likely to get a successful long lived graft with quince as an interstem than plum as an interstem.
Quince and pear are just much much more closely related. And quince is known for it’s wide graft compatibility. (EU pears, quince, loquat, medlar, etc)
Either way, getting a loquat on an existing pear tree will likely be an experiment with a reasonable chance of failure. Unless it has a quince rootstock with a side branch or sucker. Than i would estimate close to 100% chance for success.