Yeah, sometimes I go to bed without extending leases to make other players happy.
To come back to this B720 thing. A very bid disadvantage in my eyes is the instant stop of producing B707-420, the low fuel consumption variant of the B707. The other B707 variants (including B720) are nice until fuel is low, but than you will have very fast bigger problems. So if you use them, only one year leasing contracts.
Also there is a B720 (non-B) variant. Try to avoid using this model. Even with very low fuel prices it consumes much to carry 120pax 2700nm.
Another disadvantages are the four very large aircraft pilots you need.
But, as said, not this problem in Jet Age game world. And for sure not an aircraft you will fly in Modern Times.