My $0.02 is a good bike will always be a good bike. The old technology wasn't bad and high quality components will trump buzz words any day. IOW if I had the choice between 8 year old XT or X.9 or new Acera, well, it wouldn't take but a split second to go with the old stuff as long as it's not all worn out. The main problem with the 8 and 9 speed stuff is it's impossible to find decent replacement parts, so you're forced into 10 and 11 speed drivetrains to get anything now.
As far as geometry, I wouldn't think too hard over that. Yes, it matters. Absolutely so. Will she understand the little differences in a few mm different top tube or a degree more or less slack head tube? Nope. And it's not like we didn't understand bike geometry and bike fit back then. It's not like biomechanics was suddenly discovered in 2018.
Thanks again for that Dave. And I’m not really as concerned about the components availability, at least 9 speed. Performance has an XT 9 speed rear derailleur for $70; took just a minute to find it. XT 9 spd cassette is $56. 9 spd is still used on a lot of new bikes so I’m sure it will be available for a while to come.