Fair points but you have to consider that many of popular Bitcoin exchanges until recently were some nasty PHP/mysql based matching engines. The non-crypto exchanges already have the matching engine technology to easily handle volume of trades that main exchanges see today and probably with greater reliability and latency guarantees (internally). The work on the tech side would be building the infrastructure around the matching engine, clients to move crypto in and out of the exchange, supporting margin trading, cold storage of significant part of client reserves, KYC/AML auto reporting, infrastructure for top tier clients - cross connects to matching engine, trade phone support, but this isn't rocket science (although I don't downplay the work involved)