It sounds like McGuire wasn't considered a big part of the future plans, so when the Cowboys expressed interest, the Broncos were happy to do Josh a favour and let him take the deal.
It's a definitely a short term blow for the Broncos, but the staleness of the team was becoming a concern and McGuire was coming off a disappointing season.
The club will definitely miss him in the interim. I'm not sure how others see it, but I can see a scenario where the pack looks like:
8. Matt Lodge
9. Andrew McCullough
10. Joe Ofahengaue
11. Alex Glenn
12. Matt Gillett
13. Tevita Pangai Jr
14. Jaydn Su'A
15. Patrick Mago
16. David Fifita
17. Payne Haas
Or something to that extent. I can see it being a mess to begin with and fans won't be happy but when players like Carrigan & Flegler come of age it won't seem like that big a deal.
The positive is that the movie indicates that Pangai Jr could be staying. According to Chris Garry who has interviewed TPJ on numerous occassions, he's of the opinion TPJ is only signing a short-term deal due to management issues. After he's through with the one year deal, he'll re-sign for a longer deal.
The club is also looking to extend David Fifita early so they don't have to worry about all the headaches in the 2019/2020 off-season.
It's a real pity to be losing a loyal club servant like Josh. There were times where he seemed destined to be a 'good' club player, but then he'd pull off something amazing. The break he made in Origin II 2017 was a huge turning point for that series. He spoke a big game, and then he delivered.