It’s not just the length of flight, it’s the time zone difference, that Cancun flights are full of pissed-up adults drinking the bar dry, etc. Cancun itself does not have better weather or beaches than many European holiday resorts, in fact I would say the best European beaches are far nicer than Mexican beaches, for a start there is no sargassum problem in the Med.
You need to be going that far with a toddler because there is something there that you really want to experience that you can’t get closer. I think you can get excellent beaches and sunshine far closer than 10hrs away!
Im speaking from the pov of an airline employee who has dragged my children on LH trips around the world with me since they were born - not because we were going on holiday but because I was working away over key family times of the year. It’s just nicer not to have to take them so far if you have other options. Lots of people do take their babies and toddlers on LH flights with no issues, but if I had a pound for every stressed family I had with a baby on a LH flight I would be rich!
Wrt the bassinets - they are for babies up to 6 months. There is an infant seat for babies up to 2 years old. There is also no guarantee that you will get one. I regularly have more families booked on a flight asking for the bassinets than we have available. They get angry then, because they believe they have booked one, but they have simply asked for one if it is available. If we have 3 bassinets and 7 babies on board, it is simply luck of the draw if you get one. Whether you have a bassinet or not, if the seat belts are on, the child must be on your lap with the extension seat belt fitted. We switch the seatbelts on for any expected, predicted or actual turbulence. Just because the seat belt signs are on but there is no turbulence at that moment does not mean it is safe to get your baby off your lap. We frequently spend time flying around large thunderstorms, especially in the tropics, and if it is dark it is not always possible to see them. We also cannot see wake turbulence or clear air turbulence associated with jet streams, but we can talk to other aircraft who have experienced it and might switch on the signs as a precaution. We had a situation last week where we hit clear air turbulence and everything that was not secured in the cabin (trolleys, crew members, bags, etc) was lifted off the floor. Thankfully all babies were strapped in this time, but we still have passengers who don’t want to disturb their baby when they are asleep and lift them out of the bassinet to secure them. My most recent disruptive passenger incident was a very entitled mother who would not secure her child properly and was on the verge of the police being called to meet her on landing - she was convinced of ‘her rights’ and I was convinced she was not. And I was in charge, so my word was final!
If all of that still sounds like a great start to a holiday, then crack on, I won’t persuade you otherwise. I’d still be off to Greece, Turkey or Croatia, or even Spain, Italy or the south of France for a beach holiday with a toddler though!