Software is always getting updates, and larger changes which require a lot of work often have paid updates. Not sure why WP themes would be any different. Some of the stuff they've updated look like massive changes. It makes sense that they're breaking changes, and cost money.
Software is sometimes* backwards compatible. Occasionally there are changes that simply cannot be compatible, but are necessary for the good and future of the product.
Software is always getting updates, and larger changes which require a lot of work often have paid updates. Not sure why WP themes would be any different. Some of the stuff they've updated look like massive changes. It makes sense that they're breaking changes, and cost money.