These "Heroku is too expensive" comments all look at absolute costs and not relative costs. They are paying roughly $200 a month to Heroku. If you do some rough calculations to figure out revenue (use Stripe fees and make assumptions about yearly/monthly plan breakdown) you can reasonably estimate about $3,000 in monthly billings. The incremental savings couldn't possibly outweigh the time costs of switching to another provider.

If using Digital Ocean allows them to move faster, then sure that makes more sense. But doubt that's true in this case.