Do you do actual WP development? How many clients do you have? How much Yoast functionality does your plugin cover?

There are certain plugins that deliver so much value that developing and maintaining them yourself is nonsense for most devs out there. Unless your agency manages thousands of plugins and you have some very specific needs, you don't need to re-implement everything from scratch. Even then, it's probably far cheaper and easier to re-use their GPL code as the basis for a less bloated plugin rather.