Then you also need a realignment of the entire academic system as well. A new professor spending 50% of their time redoing the experiments of others, especially on capital expenditures and grad student time is going to be at a severe disadvantage against others who spent 100% of their time building their own research agenda.
We'd probably need a whole new degree that's like a PhD, but whose dissertation isn't a novel extension of the field, but reproduction of other studies. Because as it is, a grad student can spend up to a decade on a Ph.D. just doing novel research. Now you want them to spend 50% of their time on reproduction? So is a Ph.D. supposed to take up to 20 years with your proposal? Or does a dissertation contain 50% less novel research?
Certainly we should have funding for reproduction studies, but it's not enough to just have a grant; you have to have a valid career path doing such work. That doesn't exist today.
I don't think that would work - those positions would go empty. People apply for grad school because they want to do original research. On the other hand, once people are already in the academic pipeline and they run into struggles advancing from postdoc to assistant prof or maintaining funding as an assistant prof, I think a lot of people would take a 50/50 grant if lets them keep one foot in the door for original research. They aren't going to turn their noses up at the only grants they are competitive for.
You do raise a good point that labs with 50/50 grants wouldn't have as good of a value proposition for prospective grad students as labs doing 100% original research. I'm not sure what the solution is there.
They would if it doesn't make them competitive when it comes time for tenure review. Reproducing research doesn't get one tenure, so accepting such grants would mean tacitly admitting you should also start looking for a new position in a couple years when you go up for review. That's why I said we need an realignment of incentives across academia in addition to these grants, which would give a career path to people. Maybe it's worth it to create positions for people who just reproduce others' research, I don't know.