I think both IHG and Marriott convert about 5:1 to most airlines. (Hilton is even worse, I think about 6.5:1 to many airlines.)

So even if you get 70,000 IHG or Marriott points, those would only convert to 14,000 miles at most airlines, and thus be only a bit more than half a capacity-controlled domestic round trip (or, to phrase it another way, just a bit more than one capacity-controlled domestic one-way trip). So why even bother unless you've run out of all other options?

Hotel points (with the possible exception of Starwood, as noted above) are most valuable when used for hotels. Airline miles (and with a couple airlines, airline points) are most valuable when used for airlines. The only kinds of points that are valuable for transferring to airlines but don't originate in airlines are certain credit card points (for example, Chase Ultimate Rewards for United or Southwest award travel, or Amex Membership Rewards for Delta award travel).

Then there are a few credit cards that have their own points (which they sometimes call "miles") that can't be transferred to airlines but have higher value when redeemed for airline tickets (on economy travel only, but that's what you want) than when used for cashback.

But unless you have a lot of spend, the most practical way to get any of this stuff is from signup bonuses on new credit cards, not from everyday spending on cards you hold. (At 2%, it takes $20000 of spend to get just $400 in travel credits, with a typical travel-specific cashback-equivalent card.)