It's so easy to put your own interpretation on these things, and read into it what you want to read into it, and I'm about to be guilty of that when I say that what I see here
suggests to me that it was never the plan to have the Enterprise enter this series so early and the writers were simply told from above to make it happen. If that's the case then you could surmise that CBS is not totally happy with how the show has performed. That's not to say it hasn't done well, and looking at the response on this forum it certainly has been well received by many fans, but maybe not by as many as CBS had hoped.
And if I'm going to go way out in left field here and let my imagination run wild with wishful thinking, then I might want to say something like - maybe showing us Enterprise in the last episode and going into TOS theme music is really a coded message from the network telling some of us "We heard you, so just trust us and keep the faith. We will give you what you want."
And ideally what I want to see is Discovery morph into the adventures of a young Christopher Pick and the crew of the Enterprise, or maybe an Enterprise Spin off. Have that run for 5 or 6 years and then in the final episode of the final season redo "The Cage" followed by Pike's promotion off Enterprise. Then the following year reboot TOS with a Young James T. Kirk and crew, redo some of the best original episodes along with many many new ones, and take us on a true 5 year voyage of exploration and Discovery.
After that I can curl up and die happily with my bucket list complete.