If Russia sent a ton of conscripts on day 1, sure, they might have won instantly and then occupied Ukraine completely.But if they fail or get bogged down, then there is no way to replenish the army, and no way out aside from nukes.
Unless Russia sent its entire 33 million reservists and non-reservists able-bodied men into battle on day 1, I don't see how Russia would exhaust its entire conscript supply and resort to nukes. The math doesn't check out.
Also others mentioned, it's a partial mobilization, only a fraction of the 2 million reserves are called up. (Not even touching the 31 million non-reservists able-bodied men).