You would be surprised how many accountants still do that. Accountants are a finicky bunch and they don't trust what a cloud accounting platform such as Xero/QB spit out, so they end up writing their own "custom software" in excel.
They can easily expand, but they already have established competitors in that space, such as Xero and Quickbooks. Give a shot to Xero, it definitely has all the things you desire for.