I moved into a new house last year with a healthy buddleja in the garden. It was about 10-12 feet tall in its prime.
I pruned it back as per this website, and others, and it's growing back very well and as expected.
However, my neighbour has 2 buddleja trees much bigger than mine - they are proper trees, They've done nothing with them at all. Just left them at 25-30 feet tall with all the dead flower heads. Suddenly, both of theirs are in glorious bloom and look fabulous.
So why did I bother pruning mine at all?