They would still have lost.
Roger already suffered way too much because of his terminal illness and aside of the WBP, who would really help them destroying the WG? Kaido and Big Mom forces were defeated by an alliance of supernovas who barely went toe to toe with top tiers.
Joyboy was stronger than Roger and yet he lost as well.
I think Roger just knew he and his crew, including the alliance, would be washed by the combined forces of the WG + navy and the ancient weapon.
That's such a bad excuse.
"oh lets all wait for the pirate jesus to come and save everyone"
Is what it boils down to in the end.
WG didn't have Motherflame yet for Uranus.
They don't know where pluton is, but Roger pirates knew.
And Shirahoshi wasn't born yet.
So Roger pirates had the advantage on 1 ancient weapon to the WG's zero.
WG didn't have seraphs, invincible lunarian clones with strong warlord base models and paramecia fruits.
WG didn't have the countless Pacifista.
And many more of the technology vegapunk so naively built for them.
Then there's the admirals.
No Akainu Aokiji and Kizaru.
You really think if Roger and Roger pirates asked Garp to rebel against the WG with them, he wouldn't?
He would have at least stepped aside at the very least.
Roger pirates would have gotten help from WBP if Roger asked. Seeing as how they were in very good relation, and Roger even told WB about the void century stuff.
So yeah, they might've lost.
But they might not have too.
The WG was much weaker than it is now.
But they didn't even try at all.