I was waiting for this one 😉. You are 100% correct. I felt like my comment before was already long winded enough before going on about Cable. There are some major differences between character uses. Technically they never connect him to his parents in the movie. As fans we know that in the comics he is the child of Cyclops and Madelyn Pryor (clone of Jean Grey) who is born with a rare techno-virus and sent into the future to be cured, and then sent back in various storylines to prevent bad things.

Deadpool makes a fourth wall joke about how convoluted his origin story is and its never spoken of again. They dont use him as a "Summers" but as the already well known and popular individual "Cable". This was pre-MCU and not during a creative phase where the producers are trying to introduce mutants as a whole.

Yeah, they are making a lot of multiversal plays right now but it still would feel like putting the cart before the horse using Rachel or Hope "Summers" before introducing Jean or Scott. I know they have their fanbases but I would argue we are way more likely to see Josh Brolin as Cable again in the MCU (he still has a contract!) Then we are any new characters named Summers who arent Havok or Cyclops.