I'm not going to do another long post about how the number of men working a gun is a study in false economies. The issue should always be terminal effects. What do you need happening at the receiving end of the round. Not how many people man the gun.
In short, the 155 offers much greater range, good accuracy and the ability to provide significant precision and area effects. Far superior to the 105mm round in all respects.
Add to that the fact that the people on the gun provide ammunition handling capabilities, security (including CUAV/AD dets), solid 24/7 operations and can be for the most part provided with semi-skilled labour (read cheap reservists). The number of folks on the gun line is only an issue for the political staff weinies who make artificial manpower ceilings on deployments.
How many times do I have to say that with a 21 round magazine the gun will be out of action more often than it is in action for bombing up and that there needs to be a whole herd of people behind the scenes to keep it loaded, secure the area and swap out crews to provide 24/7 operation.
Folks. There's a big difference between peace-time operations on the range and war. You can operate an M777 with four guys too - just not for long. It's kind of like saying that an infantry platoon only needs two folks - one to provide covering fire while the other moves.
I'm not a naysayer on the 105. It has its uses in airborne and airmobile ops where space and weight are an issue and range isn't. For a mech brigade, you need 155 and preferably an SP with the crew under armour. The M777 too has a role in such operations
And just to prove that I'm not an old M109 curmudgeon, I firmly believe we are missing the boat on not developing a serious capability of a variety of loitering precision munitions to supplement ATGMs and indirect fire. Judging by the low amount of fast air and attack helicopter employment in the front lines in Ukraine, our JTACs would be well utilized in attacking with cheap UCAVs, loitering munitions and the like.