> You're seriously underestimating human ingenuity
Trust me, to be a scientist now is ludicrously difficult enough. I could tell you about the artificial barriers raised carefully to stop or delay for decades this people to... doh, solve OUR problems, but we would need a few hours for enumerating them.
> many young people are probably disgusted by the prospect of having to do dubiously ethical experiments on animals.
You seem to think that everybody is playing ping pong with rats in the labs but the real life is a little different than in the films. Most young people currently, right now, avoid easily this problem choosing NOT to do this kind of experiments.
You are 120% free (and strongly encouraged) to do research only in something that you like. Nobody will stop you if you want to follow a botany career. Most of the really hot stuff come from plants or invertebrates after all. Nobody will force you to do design your experiment in a non ethical way, or make illegal drugs in your lab or... whatever. You can choose.
Well... in fact it is not your choice only. Research with animals is strictly regulated and watched by independent teams. People doing such non ethical experiments would be, first: forbidden to publish in most journals and two, fired-bye-bye before 24 hours. And no more grants for you in the future, of course.
Because those animals are 1) essential for the survival of the team and a poor management would attract diseases, stopping 50 people to justify their grant money for several really stressful months. 2) extremely expensive and difficult to obtain. 3) bond with their caretakers more that you think and 4) Its sacrifice could save thousands of lives and sustain thousands of jobs.
And because universities and companies will avoid bad publicity at any cost, no matter how student heads will be necessary to chop.