The 2 roles are not mutually exclusive.
"Gatekeeping" the rationality will consist not only developing but also imposing the appraiser's judgement on the actions of all these market participants. Judging the actual actions of those buyers and sellers writ large. Picking winners and excluding the losers. Acting with a bias that, quite honestly, is based on a predetermined conclusion.
That isn't part of the Prime Directive of a MV opinion.
Now that's not to say an appraiser cannot or even should not comment on the sustainability of a trend or where the pricing fits in with local incomes or past pricing. But as far as picking comps in order to indicate to the dominant trend in the market goes, "what's sustainable" isn't part of that math.
For example, if I wanted to tell the lender that I don't think the prices that I'm observing in the market will hold I could just say so without it affecting my comp selection.
The highest priced home ever sold through the MLS in this area prior to the current RE cycle was a 2005 sale of a then-new home for $600,000. The median pricing at that time was $405,000, which at the time amounted to 9.8x the median household income. Since then the top pricing declined to $340k in 2011 with a subsequent rebound starting in 2012.
More recently, the high sale was in 06/2021 at $720,000, with the current median at $570k, this amounts to 7.9x median household income. The 2020 median was $500k which was 7.0x median household income. IN MY OPINION, the current annual rate of increase of the median pricing of 12%/year is not sustainable over the long term. However, it's unknown when or if the current trend will flatten or reverse.
There. You just informed the reader of where the current pricing is in relation to the incomes and conveyed your opinion about the uncertainty of the current value trend. If they proceed with maxing out the LTV based on your current appraised value but suffer a loss later on it won't be because you somehow obscured what's going on in the market. If anything, you basically warned them of their risks and they went ahead and made that deal anyway.