How do you modernize a legacy tech stack without a complete rewrite?

As everyone warns about rewrite projects that they are set for failure, how would you modernize legacy software written with an out-of-date tech stack like Visual FoxPro or Visual Basic 6 without a complete rewrite?

We have a lot of internal applications written in those tech stacks (FoxPro, VB6, ASP, etc.). Everyone seems to say that the right way to modernize these software is through the strangler fig pattern, but how would it work with these tech stacks where the new and old software can't co-exist?

We are starting a migration project to migrate the largest internal application, migrating from VB6 on Windows to a web-based application backed by Go. Everyone on the team agrees that a Big Bang rollout is the only way. Curious on what you think.

More background here: https://www.reddit.com/r/programming/comments/1piasie/comment/nt4spcg/