That would be like the Russians pouring into Alaska because we “stole” Alaska from them.
Or the Louisiana Purchase of 1803, "stolen" from France for about $15 million including cancellation of French debts. In California, the Russians told the Spanish they weren't interested in keeping northern California, so they walked away from it (Fort Ross in Sonoma County just north of SF, still there). Russians sold it to John Sutter (of Gold Rush fame) for practically nothing while Sutter was under Mexican rule. The Russians got a little smarter later on and sold Alaska to the USA for real money.
So, if we give the west including California back to Mexico, we should give Alaska back to Russia, and the Louisiana Purchase comprising 15 US states back to France. What would we have left as the USA? We bought California and own it; we should have made Mexico a territory when we won the war against them, at least they would have a legal basis for crossing the border!