we have new release with new features each 2 month. and canary release each 2 weeks. there are ways to put something new not breaking everything around. because if you do, not only your costs for testing will increase but also your costs for development, planning, documentation. and those costs will be much more serious. I dont know how you can explain to your customer something like "ok, you want to add some new feature, so we also have to rewrite all other features. it will cost all that you already spent and little more."