It certainly doesn't seem that Legolas had a vast devotion to the Woodland Realm. He comes across as very impressed by the outside world, Lorien of course, and then Ithilien, to which he moves with a company from Mirkwood, before leaving Middle-earth (including his father and people)altogether. Still, I suppose this doesn't necessarily mean he wasn't the eldest son. Being an heir wasn't so important in an immortal race. There was no reason for Legolas to expect to outlast Thranduil, barring accidents, which of course might just as easily affect him I count wars and battles as accidents in this context. (It has always seemed to me unnecessary to think too much about heirship, inheritance and lines of succession being all that important to elves because of this fact.)
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