Haven't read all the posts on here, but I would like to share my Opinion on some points raised.
Instructions & fit: This will depend on how much money and research time the company has to spend on it.
Tamiya as an example often gets the real thing into the workshop and they design the Instructions and kit from the original.
Other makers work either of drawing, specs or only photographs.
The lates Tamiya planes are nothing more than repackaged Italeri Models and thus NOT Tamiya quality.
For many companies it also costs a LOT of money to get the Instructions translated and the cost of issuing multiple Instructions does not generate them a profit.
Aftersale parts and upgrade parts:
Those, IMO, are for the experienced modellers & enthusiasts who already got quiet a few models under their belt, not for the young ones or the inexperienced modeller.
Many of those parts are not ready-to-use out of the package and will need care in trimming and aligning on the Kit for which it is designed for.
Colours:
Most Makers cannot afford to stock 100's of shades of colours to conform to every model of theirs, so they eitehr:
a) Give a colour of theirs close to the original
b) Give a ratio on how to mix a colour close to the original colour.
BTW, what happened to the IPMS colour guide and the paint chip books, used to be the only way to get correct colour matches.
Having been around when Plane models consisted of less than 10parts, came in a plastic bag with the Instructions on the sealing flap I must say that on average the quality of Models has greatly improved.
Remember the articles on how to cut your own accurate panel lines on Planes and so on, or that back in the day 90% of all Plane Models shared the same Cockpit instrumentation.
Granted there are some companies that haven't really upped their game for the last few decades, and there are some that are literally strapped for cash and cannot afford to offer better quality but instead offer Kits of rarer items.
Make a high quality accurate Kit it will be beyond the means of the average modeller.
IMO, it is impossible to make a model and set of instructions that will satisfy everybody.
Some like me hardly ever look at the Insturction sheets and decide on our own colours.
Once build an Academy(Tamiya) Bradley M2 with interrior without Instruction sheet as it was missing, that was when Academy still offered cheap pirates of Tamiya Kits.
For younger Kids I still recommend a few simply plane models, snap-together kits and so on.
FWIW, I am NOT going to give my Opinion about the skill level systems offered by many Companies and Online shops.
I think the best we can do is keep ourselves informed as to which Company makes the type/quality of Model we enjoy.
Example: I prefer the Revell Germany Kits but couldn't be bothered with Revell USA kits.
Personally, I think that the Model Hobby has split into 2 distinct directions:
1.) Highly accurate Kits for the experienced modeller
2.) Easy to finish Kits fo the Kids and people that want to "play" with those toys. (Gundam, etc)