Look at the Discovery, the crew uniforms, the bridge design, etc. How are you going to marry that up with TOS-era Enterprise? How could you do a Trek series in 2017 and make everything styled/dressed like 60's Trek just so when the crews meet, they match the established old look? You can't do it. Discovery had to be brought up to date (even though it takes place before canon TOS), and if Enterprise was to appear in it, then the ship and crew has to be updated too. It's a telling of a story we've never seen from that time in the Trek universe, it doesn't have to look identical, it just has to tell a good story. In fact, the story being told fifty years later has to be visually updated, or it wouldn't be accepted and people wouldn't watch it.