The domain is mutual fund accounting. The system in question had grown organically over decades to encompass everything related to mutual funds; account record keeping, shareholder statements, broker dealer recording keeping, commission payments, cost accounting, you name it, they do it.
I agree it is an oppurtunity, but the barrier to entry is very high. Reaching feature parity is a multi year project with a large team and domain experts.
Sorry, I did not realize you meant to create a new software from scratch. I thought you were talking about supporting/extending/refactoring this old pile of madness. Thanks for the clarification!
I suppose you were referring to the statement "Customers in that space don't have many options". Your statement is reasonable and there might be market opportunities to create new software. But I somehow have the feeling that it would take a tremendous amount of time to recreate something that checks the same boxes as the old system. But I do not work in that sector so my judgement might be totally off.
Thing is, a competing product does not (at launch) have to match the entrenched one feature-for-feature. It just has to tick the MOST IMPORTANT boxes while being advantageous in other ways (such as being built on FAR newer tech, more reliable etc.)
Being built on newer tech isn't an advantage from a customer POV. It's only an advantage when the newer tech is better, and tbh I have a hard time believing that you'd match the performance and security of a mainframe that easily.