We don't need to endlessly increase productivity. The US been increasing productivity faster than wages for decades and it hasn't been good for most people.
I'm not arguing either way for whether "we" need to as a whole, but if you want to get the most out of your team and company (and you should if you're running a startup) I think you should look at in person options again once things are somewhat normal if you want an advantage. Some sort of hybrid office(with dedicated per employee offices)+WFH flexibility is probably your best option.
No. There's not a lot of research into it yet because the ability to work from home is relatively new, but this is literally my wife's area of academic expertise and the research that does exist generally shows that given the proper conditions and environment, working from home can be vastly more productive.