Ask HN: Integrate with legacy back office?

A lot of "traditional" companies want to develop modern solutions (mobile apps and web applications) because they are seeing all kind of tech startups entering their business domain with very user friendly user interfaces.

But these fresh startups don't have any back office applications that were implemented 20 years ago. They also don't need to support a current customer base etc. Actually, a lot of these startups have the dream to be in that position one day.

The dilemma that these "traditional" companies face is: should these new solutions be integrated with the legacy back offices or should they be stand alone?

Both choices have big advantages and disadvantages, for example if chosen for legacy system integration: - It will probably slow down the development of the new solution significantly, because these legacy systems don't have proper APIs, - No knowledge about the systems because people that built the system are gone, - Are monoliths and is difficult (impossible) to change something - Based on old fashioned business process - Performance issues - Will maybe not feel like really a coherent solution (integration will leak)

+ Don't need to build all kind of functionality like generating an invoice or managing, inventory. The new devs can focus only on a small scope, + No need for training people to work with new back office applications, + The concerns are separated, everyone can focus on a defined area

Should these companies start creating a new fresh stack because it will never work with the old stuff? Or should it be a more phased transition per application? Or there aren't any general patterns, and should it be analyzed always per case? What are your experiences and thoughts?