"Alternative mainlines" really need not directly follow the PF model of "Starts worse, becomes better late". There's room for two mainlines to provide "Different" strengths to an army, rather than one simply being "better" than the other. I could well see Penals being made to be a more "Aggressive" Mainline, and Conscripts being tweaked to be more "Defensive", for example. The specifics involved in this are debatable, but I really don't think there's cause to dismiss the concept out of hand.
.
if both units were exactly the same except for vet we would see this already. con vet makes them very durable and penal vet quite formidable.
one might entertain the idea of a soviet infantry system reform
consider:
concripts- t0. cheaper and worse than now. something more akin to ostroppen. only good in cover but swarm the field- no vet.
7 man with t3 but no bonuses to cooldown. expendable
when t1/t2 is built they can be "upgraded" via promotion or demotion...
strelky- promotion upgrade path of cons. more or less what cons are now.
can be upgraded with a DP-28 when t3 is built and another at t4. DEFENSIVE infantry.
penal battalion- demotion upgrade class of cons. instead of defensive vet, they got penals offensive vet.
can upgrade to 3 SVTs with t3, 3 more with t4. OFFENSIVE infantry
both stem from the same core, t0 unit. both paths cost resources. both paths lose access to merge, oorah and sandbags so cons are always useful
if one wanted to take it further, once fully vetted these paths could continue into elite infantry. promotion path could culminate in guards at t4 instead of an extra dp28 (requires upgrade) and the demotion path could end in shock troops (again, requiring upgrade)
rough and ugly concept but eh.
gives soviet a unique feel and a diverse infantry roster all based out of conscripts.