Well, there are three points on that:

• Business investment doesn’t need money from the rich, it just needs any money, resources and manpower. Shareholding means lots of non-rich can supply the money (it’s not the rich that are needed, it’s the money) and structures like Cooperatives mean that many businesses can be made by people just pooling their work and resources. Theoretically at least mass Shareholding should make for a more robust business environments because many people investing should have a lot more variety (hence making the whole more resilience to the kind of unforeseen changes that cause Crashes) in terms of what’s invested in and the investment objectives than just a few people.
• The money being too concentrated together with the current Ownership Laws (mainly Land Ownership, though in some areas also Intelectual Property) actually crowd out most news businesses because of how expensive it is to launch most business ventures, not just directly but even in terms of the founders themselves being able to afford being out of work whilst they launch a business. Notice how even in big cities but especially in smaller cities and even towns, stores are closing and those spaces remain closed for months or even years. The money and property concentrated in fewer hand has the leverage to demand huge rents from the rest of Society to be able to use those thing they’ve locked-in through ownership and that’s killing lots of business at the start stage and even stopping the business ideas themselves from ever being put in practice. It’s “funny” that the rich having all the money creates a situation were so much money is needed to launch a successful business that it can only work with funding from the rich - nobody is going to create, say, a large restaurant chain from the humble beginning of a single venue in a small town when the necessary realestate to expand or even just start costs many times more to rent or buy than it did back in the 60s and 70s when so many of todays big name such chains started just like that.
• The actual value of more investment depends on were the bottleneck is in the Economy: supply side or consumer side. There is no point in adding more businesses (i.e. Production) if there’s a lack of demand (i.e. Consumption) because median incomes are too low. If you look around (just notice companies nickel & diming customers) we currently have a lack of demand, not of supply, so money going into investment just makes the problem worse whilst money going (via better wages) into consumption would help.