The option to add a 2nd trampoline hop to legacy trampoline payments requires too much space in the trampoline onion.
Because the trampoline onion has only limited space of 400b and the payload with 2nd t hop and no r_tags already requires 392b it is essentially unusable for payments that require routing hints to be included in the onion
(because the receiver has no direct channel to the 2nd trampoline node)
as there is no space left to include them (1r_tag == ~52b).
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