
Pineapple liqueur is a sweet fruit liqueur flavored with pineapple on a neutral spirit base.
The liqueur carries pineapple, sugar and about 20% ABV in one pour. Any stand-in covers some of that, and the rest has to come from somewhere else.
Fruit and sugar without the alcohol. Use it where the drink needs pineapple but already has its spirit. Make the strength up with neutral spirit, roughly on par with the syrup.
Weaker, sharper, and it lengthens the drink instead of sweetening it; put the sugar back with syrup; the foam in the shake comes as a bonus.
Pineapple aroma in a bottle is a set of esters, and esters go. The sweetness and the acidity stay put, so an old bottle is still usable and simply less specific. Cool and closed, out of the light.