There is a very small increase in endometrial hyperplasia risk when sequential is used long term, however this can be avoided by having an annual private ultrasound scan to check your lining remains healthy.
The NHS hasn't the resources to provide this level of individualised care so have to err on the side of caution and have everyone on high dose continuous progestogen.
Whilst this does avoid endometrial cancers, the trade off is many women being over treated with progestogens, which results in other negative outcomes.