Mixed fleets that do not need long term sustainment are radically different from fleets that need lifecycle management, and the associated costs and complexity they bring, which pulls more personnel (an expensive asset, whether in or out of uniform) into managing spares, conducting maintenance, ammunition storage and acquisition...
EDIT: What Infanteer said.
I, and we all, appreciate that. But it's a philosophical difference (and I'm not striving to make this another shared fleet debate)
The issue is does the Army want to: a) equip two of three brigades pretty well? or does it want or b) equip two of three brigades well and another 2 brigades less well?
When the Army decide that a given in-service fleet should be replaced, it already has an existing lifecycle management structure in place for that including a parts system and trained personnel to maintain it. It's the new system that requires adding a new lifecycle management structure.
While older systems generally become more maintenance intensive over time, repurposing an old fleet from active to reserve status means it will have much reduced usage which provides a lower rate of deterioration and more time for maintenance. If one built in an active/reserve lifecycle to the acquisition of any equipment it would ensure that at the end of its active usefulness the equipment would still have a measure of useful reserve lifetime.
The Army don't think like that, however. It use the hell out of gear and then dispose of it to eliminate associated maintenance costs. Worse yet, every time that the Army buy new equipment it not only dispose of the old but generally buys less of the new than it had of the old putting the system into a slow death spiral.
Many countries have a "hand-me-down" approach to ensure that the various parts of their armies have equipment when needed. Canada seems content to have an army that has over half of its personnel unequipped or underequipped with major weapon and vehicle systems. It's not rocket science to establish supply and maintenance systems that can support such fleets. Yes there is a cost to that, but its a cost that results in a more extensive defence capability.
Well. I'm off to rebuild my front step. Using ice melt last winter damaged some of the concrete blocks ... which is good because I can keep the half which are still serviceable.