Eventually Epic may be replaced but it's really hard to break into the hospital market. Any new competitor would have to meet all of the ONC Health IT Certification requirements, plus a bunch of other checklist requirements imposed by hospital purchasing departments. It doesn't matter whether they solve the UI problems or offer new capabilities if they don't have the basics finished first, and that takes many person years of work.

https://www.healthit.gov/topic/certification-ehrs/certificat...

The only way a startup might be able to eventually replace Epic is to target a niche ambulatory care specialty first where meeting all the checklist requirements is less important than really nailing an optimal clinician workflow. Then gradually expand out from that foothold by adding the features that hospitals need.