You seem to be basing your argument on a simplistic and inaccurate view of how the US military is actually structured.
In the wake of WWII we started the move to establish what are called Unified Combatant Commands. That general concept morphed along the way until the establishment of the current system in the 80s. UCCs are not just wartime commands. The exist in both war and peace to be responsible for a specific geographic or functional area. (US Special Operations Command is an example of a functional UCC with worldwide missions.) Since the last major change in the 80s the UCCs dont fall under the Joint Chiefs or the service departments within DOD. Their commanders take orders directly from National Command Authority which is by law the combination of the President and Secretary of Defense. The UCCs are joint.
UCCs can establish mission specific joint headquarters called, without creativity, sub-unified commands. Those are also joint. US Forces Korea is a good example because it avoids most of the complexity that comes with typical coalition command structures formed with our allies and partners. There can still be bits of weirdness like special operations forces in Afghanistan since 9-11 that can cause friction. Some of those special operations forces have operated directly USSOCOM and some have been assigned to the US Central Command (CENTCOM) sub-unified command in Afghanistan.
Then there is where the bulk of forces in those UCCs fall still to this day. They are in service component commands. Those are mostly service specific organizations under the UCC. Without digging through each COCOM for changes, that is how all of our geographic UCCs are organized. Those units that you think of as joint, because at the four star level they are, aren’t very joint from the three star level down. Unless assigned to an established joint sub-unified command those units train and operate non-jointly until task organized for specific missions that fall below the threshold of establishing a sub-unified command. In EUCOM (US European Command) the army brigades fall under USAREUR (US Army Europe) on a day to day basis.
Then there are forces that don’t fall under any of the UCCs. A great example of that is US Army Forces Command. It currently consists of about 780k Soldiers from all three Army components. That is out of a little over a million Soldiers in the total army force. The role of FORSCOM is to provide trained and ready forces to the geographic UCCs as needed. There are comparable organizations for the Air Force, Navy, and Marines. There are also organizations in the various services that perform other important service specific institutional tasks like Training and Doctrine Command and Army Material Command within the US Army. Again those are non-joint organizations that don’t work directly for a Combatant Commander.
We still mostly have forces under service specific command. We don’t just do things this way because of institutional inertia, although those forces can be easier to identify. There are real benefits to having interoperable services that are ready to fight and fight well together. There are also real benefits to having subject matter expertise to train, prepare, and equip units within in their specialty. You cant maximize both. There are costs and benefits associated with every place on the continuum from wholly unified military to a wholly service specific military. We have been on a historical trend away from a service specific model near one extreme towards a balance. That is not the same as drawing a lesson that the best solution is necessarily continuing that trend off to the opposite extreme of a wholly unified military.
There is also some balancing we needed to look at in how we deal with the growing importance of space in military operations. We tried a couple different organizations to knit together the space focused forces across all the services since 1985. The was a subunified space command.There was even an Air Force service specific major command focused on space before that point. Creating a separate service as an option predates Trump running for president. Trying to organize in a way that knits together the relatively low density of forces working in the domain is important. What we had wasn’t necessarily getting the job done well when run through existing intra and inter service rivalries. Trying to have a good match between the power of the organization to fight those battles and the importance of the mission mattered.
It looks to me like we didn’t hit a good balance point. It is hard to tell because we didn’t have a good public, but mostly ignored, discussion of how to do better. Instead we got a deeply unpopular president weighing in publicly with a position about a discussion most were ignorant was even going on. The usual suspects started dominating the public discussion in a great example of the Dunning-Krueger effect. It was either the best idea since sliced bread or an idea as idiotic as suck starting a jammed pistol depending on partisan loyalties.
I was leaning towards moving space from a sub-unified command under US Strategic Command to a full UCC based on what I had seen. It certainly seemed like it might be time to try a change from what wasn’t working all that well. A UCC like a good balance between increasing their organizational power and not adding too many extra inefficiencies. It is not nearly as simple as the OP though.