I specifically refer to the spells Infernal Calling and Summon Greater Demon. Unlike the spells "Conjure Elemental" and "Conjure-Fey" where the monster remains for the remainder of the duration when you reduce the concentration and do not disappear. This gives you just enough time to try a planar binding. The new spells cause the devil and demon to disappear within a few seconds to minutes as you lose concentration, and you have no way to keep them there.
The only thing a magician can do in 5e is Elementals until they get Gate, unless they are called by another caster to summon the monster they want for them, unlike the magician's insularity does not hurt and does not make much sense to me. Am I missing something or is this basically the only solution, except to go with the desired creature in an airplane and capture there?