Usually the threats have to do with withholding of support. The promise of support is not credible. They would rather have Trump lose with Haley as VP so she will be positioned for ‘28, than risk their whole edifice be toppled. No deal!
If the GOP want Haley to be the frontrunner for 2028, putting her on a losing ticket is the worst idea imaginable.
Losing presidential candidates rarely get another shot at winning a presidential election. Being a VP candidate on losing presidential ticket means the end of your political career. Just go back over the last 40+ years and see how that has played out:
1984 -- Geraldine Ferraro (D)
1988 -- Lloyd Bentsen (D)
1992 -- Dan Quayle (R)
1996 -- Jack Kemp (R)
2000 -- Joe Lieberman (D)
2004 -- John Edwards (D)
2008 -- Sarah Palin (R)
2012 -- Paul Ryan (R)
2016 -- Tim Kaine (D)
2020 -- Mike Pence (R)
Just look at that list and think about what happened to these people after they were on a losing ticket in a presidential election. You'd be hard pressed to find a more insufferable bunch of misfits and losers on the national political scene.