CO has already stated that it does not plan to raise the top tier above 75K. Who knows, it could change. In terms of earning miles - CO has equalized the mileage bonus to 100% for top, and 2nd tier - same as United. The only difference then becomes upgrade priorities and some savings in fees. But with no reciprocal upgrades between CO and UA then the mileage to earn top tier does not matter between the airlines.

So in terms of earning miles to redeem - CO and UA are the same: 100% for tiers 1 and 2, and 25% for tiers 3. With 50K requirement for 2nd tier the same, minimum amount of flying to earn the 100% bonus is the same.

I would expect an equalization of redemption levels. The only issue I see is international lounge access for mid-tier on CO.