I can confirm, having let my subscription expire, that your tree will remain intact, even if it is larger than the tree size allowed for non-paid subscribers. But as cornishwalker says, you won't be able to add to it.
You may receive several emails offering subscriptions at a slight discount. After ignoring them for over 6 months, MH offered me their top-tier subscription for less than the price of the basic version, so I am currently a member again for now

Edit: your post just reminded me to cancel my renewal, and on doing so I was immediately offered a 60% discount on the next renewal, the discounted price being £91.60 (down from £229). But by ignoring their emails as above, I paid £57 for the same subscription last year.