What do you mean by "a locality" in your original post? Does it mean a municipality or a development site? That is, are you commenting on comprehensive plans and their background studies prepared by other municipalities or agencies; or are you commenting on commercial rezoning applications?
Scale is an important consideration here. Unless your work deals with comnprehensive plans over large areas, it is really not worth spending the time on anything other than using simple analyses as you had described.
In our planning works with my communicty, we do not usually use anything other than simple analysis on commercial land requirement. There was one instance, however, that I used the "Market Study" approach to estimate retail commercial land requirements. That was done for a settlement of 20,000 population.
Basially the method was to estimate the retail sales potential for that community. Convert that into square footage using per footage sales figures. Then deduct the existing retail commercial sqaure footage of that community from the estimated total. If there is a significant difference, then convert that to land area.
But as you can see, it is quite time consuming unless you have a master spreadsheet already set up. It also require a lot of raw data and making many assumptions along the way. The end result may not necessarily be more accurate than using a simple line projection method.
As a test, you can try both methods on the same situation.