There will still be entrepreneurs who discover profit opportunities and people with bitcoin reserves to help with funding.

Fixed money supply doesn't mean there's no economic growth. If a venture expects to make a profit, then they could afford to pay back a loan with interest.

I think you're probably imagining something like money vanishing after a loan is paid, rather than it going back into circulation. You could pay back a loan with the same bitcoin over and over again, if you happened to be earning it back between payments.