Moving to strictly self-propelled has its merits as long as we understand there we limiting some of or options such as air-mobile ops etc. Also all SPs have a higher profile so cam and concealment options change. And maint increases also. As for getting rid of the M109? Guns are never obsolete in the fact that they all can throw a round down range which is our function any ways. The only changes actually required are in fire control (both CP and gun end) and range. The 109s have both. The only reason to change them is to move away from tracked vehs.
As to moving to 105 SPs I don't think you are gaining anything unless moving to 155s. The only advantage I see is rate of fire.
However moving to a wheeled 155 SP ( the South Africans have an excellent version - can't remember the model) would fit in nicely with the change of doctrine to a wheeled army (ie LAV IIIs. Coyotes etc.)
Maybe it is time to re-think things and seriously think about this option
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Ubique