I don't know where you're getting your information from. "Hamsters" is actually a misleading term because there is only one hamster on Earth and he is immortal. He travels through time and overlaps his own timeline frequently, creating the illusion of multiple hamsters. As he ages, he grows very slowly and the patterns on his fur change, which is why he looks different sometimes. You should probably say a few hail hamsters now because he doesn't like when people make jokes about his demise — and he can bite.

Well, that doesn't mean it's not done.

My sister had a few (more than 20 simultaneously) rats.

One rat got stuck with her tail in the cage, and the tail had to be amputated, or the rat would've died. There were a few complications during the operation, and the bill would have skyrocketed to more than 2k€. For a tiny rat.

Needless to say, my sister insisted the operation continue. Rat lived happily for another 18 months

Why not for research? I could imagine there are some very specific research projects that could make that necessary or interesting enough.

"economic viability" doesn't apply when you love something.