Why would anyone want to share a staff of people who are invested in the carefully constructed image of a trio, who were ultimately loyal to the future King and Queen and the establishment that paid their bills and gave them prestige, and who were disrupted, personally and professionally by a decision to be a couple, when they couldn’t control the dynamic?
But here's the thing - and this blame should be squarely placed on Harry, not Meghan - they (the Sussexes) were never going to be in a position to "control the dynamic." Whether they continued to share a staff with the Cambridges is irrelevant - Prince Charles was funding both the Cambridges and the Sussexes with the Duchy of Cornwall revenues while he was POW. When they moved their offices over to Buckingham Palace, they were then under the control of the Queen herself. It's not entirely clear whether the staff was being paid by the Duchy of Lancaster (the Queen's estate/holdings) or the Duchy of Cornwall, but none of it was being paid for by the Sussexes themselves - and it never would have been had they stayed. Once Charles became King, it's certain they would have been funded by the Lancaster monies while the Cambridges (William as the new POW) got control of the Cornwall monies.
Additionally, in serving the monarchy as a working royal, you are never, unless you're the monarch, going to be the one calling the shots. It really is, fairly or unfairly, a huge sacrifice of autonomy and control - especially over any public narrative. You are there to serve in whatever capacity.
If Harry didn't "understand" how that works or didn't explain it fully enough to Meghan or give her the chance to experience it close-up in full technicolor before they got engaged, that's on him. It's also on her, for not properly doing her homework or, if she did do that homework, arrogantly thinking she could change any of that dynamic as the wife of a second son.
Regarding the thought that the Trio were happy and content - that's the outward picture we saw.
Harry had to feel the outsider. You can't (at least in my experience) have a good relationship when you are the single. You are always going to be the +one for everything. And two will always go off without you at the end of the day.
The notion that Meghan broke up the trio is really stupid. Someone was going to eventually enter into it, unless you wanted Harry to be the lone wolf for life.
If you've read Spare then this up above was basically confirmed by Harry himself. Behind the garden walls, he was a lonely man living in a small cottage looking across the way to the warm, homey lights of the Cambridges' expansive and luxurious Kensington Palace apartment.
In retrospect, it was a mistake, once William got married, to continue the Royal Foundation of PW & PH and keep their communications staff together. Hindsight is 20/20 but Charles should have moved Harry's office back to Clarence House and impressed upon him at that time that his role and position was vastly different than William's. Perhaps that would have prevented or at least decreased the notion that he and his wife would ever be in a position to control any dynamic.