Having had a good look at the chainwheel spockets, all the teeth look to be still ok. I wonder if the bicycle has not been used much considering its age, or certainly well looked after.
I can see your point about probably only needing the outer and middle front gears rings.
Most adult size bikes get bought new by people who had good intentions about getting more exercise, then after a few rides and the realisation that cycling against headwinds and up hills can be pretty hard work, they often get put away and rarely used, and their owners revert to their previous inactive lifestyle. It's not at all unusual to find bikes that are 25 or 30 years old with very little mechanical wear on them, sometimes even still fitted with their original tyres.
I don't like modern bikes as I consider them ugly, so I only buy secondhand steel ones from the 1990's and earlier, and I always try to pick up mechanically sound bargains that have been gathering dust in sheds and garages. I have a couple of quite expensive model Raleighs from the 80's that are to all intents and purposes, still almost new with negligible wear & tear.
To clarify gear use, I will use all three front chainrings on my triple-equipped bikes, but the small one is only ever used for short steep climbs, especially if I have to stop at a traffic light on a noticeable gradient. My usage of the large and middle rings is roughly 50/50 and the small ring probably less than 1% of my riding. It's nice to have though in some situations and it makes sense to use it where appropriate. The only bad thing about granny rings is you have low chain tension when using the really small gears, so a chain is more likely to come off than when using the larger ones.
