I do think that only children, especially of older parents, can end up with a lonely and burdensome middle age, when having sole responsibility for decrepit parents can coincide with their own 'parent of young dcs' years.
Even if siblings aren't close, it's less isolating to have someone else to share and discuss burdensome family stuff with (can still fall to one and cause resentment of course but on average, I think sharing happens and helps).
Selfishness, sharing etc I think has a lot to do with personality, as well as upbringing. Not all onlies are spoilt. Some are good at throwing themselves into groups of dcs, others are a bit 'semi-adult', good at talking to grown-ups but bad at 'mucking in' easily and with other dcs.
The most selfish, ruthless individuals I've met, with the biggest sense of specialness, entitlement and greatest willingness to trample on others to get what they want, have been middle children. I'd put it down to nature rather than nurture for both of them.