How do you measure Retention in the HEART metrics with B2B enterprise software?

When working with B2B enterprise software that is often sold in 5+ year contracts, how do you (or do you) effectively measure retention?

The end users are employees of a company that purchased this software and they do not have an alternative readily available. This situation would skew typical retention results.

Or is it best to skip end user retention and simply focus on the larger business's ability to retain the client over the long term?