Replace lost units with tanks in late game?

Often times in a round, occasionally I will purposefully kill off units in late game. For example, IMO mortars lose their vitality in the late game, when superior artillery units become available, since I often pick the LFH18, I find 1 LFH18 15 popcap, to be more viable than 2 mortars requiring 12 popcap.

Or lets say my opponents kills all my gren squads, in a massive Katyusha/Land Mattress/Caliope barrage. Well grens are hard countered by these units, they don't stand up well to these barrages. So I figure it's best to lose them all together and not invest more mp/resources in keeping the grens alive or equipping new squads with MG42's, and just transition to vehicles which aren't hard countered by the Katyusha/Land Mattress/Caliope, like the StuG for example if I need AT, of the P4/Brumbar if I need AI (with a slight amount of AT as well).

Usually I will then pop out a 3rd pioneer, and then just mainly focus my army on tanks, and keeping them repaired and such.

I just find that vehicles don't produce MP Bleed, they're often cheaper overall mp-wise, the fuel costs of them in a way saves mp both in being slightly cheaper and performing the same function as a pure MP unit, and no MP bleed.

On the other hand, if I'm floating a lot of MP late game, and am a bit short on fuel, and I desperately need an AT unit, and I have plenty of munitions, sometimes I will go with say 2 squads of Pgrens, since Stug's are hard countered by other tanks like the Firefly/SU85/M36, however the Pgren is not.

Usually when deciding to purchase a unit, I look at my MP/Muni/Fuel reserves. If one is higher than the other, that dictates what the next unit I need will be.

On the other hand, since MP is crucial for purchasing all units, if I'm light on those, and need say, AT, I will just lay Teller Mines if I'm floating some Muni.

Do you guys also replace lost units in the late game, once popcap is freed up, with superior late game units?

I'd say it's entirely based on what your opponets composition is and whether or not you're trying to force him into decisions. I can get 10 USF 75MM Shermans, and it'll force my opponet into mass AT. But with this build you don't have a form of infantry that can effectivly deal with his infantry and his AT weaponry at the same time. Tanks are also heavily countered by mines, infantry not as much (exlcuding ostheer S-mine)
Also don't forgot you need infantry to capture territory. Less with USF because vehicles crews but still the crews can't deal with fully vetted Axis infantry.

Short version is generally you want to aim for a well rounded composition so you can deal with all threats you may come up against. If you're heavy on one end of your composition your opponet can exploit it.

Smoke barrage from mortars always useful even late game.

In team games you could dedicate your army to a role of tanks or artillery or something of that nature as long as your teammate is capping points then it's ok.. You said opponents so ill assume you mean team games.

I do replace lost infantry units with tanks in the late game in 2v2 - way more often with Axis than with Allies though. If I have lost my entire Grenadier and team weapon force to Calliope/Mattress/oneshot wipes/etc. I usually stick to Pioneer spam to repair my armour as fast as possible. Since armour is my only remaining attacking force at this stage of the game I think it is most important to grant it permanent operational readiness.