Depends on the datnoid. Some will become more confident when they get bigger and others won't. If they're stable now, odds are they will stay that way. If they are unstable now, it will be hit or miss.
I wouldn't say most Indo's won't be stable T1. I have 7 and 5 are stable almost 24/7. ST's cost more for the reasons stated but most likely it has to do with the fact that they are extinct in the wild. Supply and demand.
Depends on level of stability your looking for. If you want your dat to eventually end up with a jet black bars with a golden base you might want to get a true pulcher or thinbar. Monster healthy indos usually end up with dark brown to light brown bars for most of their adulthood. Many small indos have jet black bars due to debatable factors
I had a traditional stripe indo that was stable in my tank for 4 months at 7" then it got to 10" and went unstable for 2 years and grew to 16"
I then sold it and it has been stable for nearly 1 year in a different tank
As for ST being so high priced well I think if you get a perfect 3 bar which is stable has a good body shape I would take that any day over a forked bar ST with messy stripes and not the same on both sides
People just want ST just for the sack of it even if it looks rubbish
To much focus on ST or IT you should price a tiger on what it looks like not what it is other yes what they are matters but not a fish like a tiger when most people can't tell if its IT or ST