dnd 5e – Is there no way that a lone pitcher can unite a demon / demon with the spells of Xanathar?

I refer specifically to the Hell Called and Summon Greater Demon spells. Unlike the spells Conjure elemental and Conjure fey, which make the monster stay for the rest of the duration when you lose concentration instead of disappearing, which gives you enough time to try a planar link, the new spells make The Devil and the Devil disappear in a few seconds or minutes as soon as you decrease concentration, and you have no way of keeping them there.

The only thing a conjuring magician can tie in 5e are the elementals until they get the Gate, unless they receive the help of another launcher to summon the monster they want for them, which goes against the normally insular attitude of the magicians And it does not make much sense to me. Am I missing something or is it basically the only solution besides going on a plane with the creature you want and trying to capture one there?