Sounds nice in theory, but won't you always be playing catch-up? Or will development(/bugfixes) halt on the existing python version? Would it ever be acceptable

Sounds nice in theory, but won't you always be playing catch-up? Or will development(/bugfixes) halt on the existing python version? Would it ever be acceptable to the business to commence such a rewrite without any show of value until after completion? (Speaking as someone working at a firm that did choose (before my time) to start a ground-up rewrite than ran about 3 years over estimate).