Look, you are not getting it
You can tweak the land value, to match the market approach. The point of a cost approach is to develop a credible opinion of value, and break down that value rationally. To adjust the value to become more aligned with the market approach is not unethical, or an unprofessional move. You are putting to much into this " back into the cost approach" thinking. You are downing my opinion, without offering any credible alternative methods. An alarmist attitude at best. The cost approach helps draw benchmarks for land vs home valuation for each specific assignment. What should your benchmark be for land, if not a market derived value? How does this affect the current market valuations? To adjust the land value is appropriate, to some extent. One poster implied that you should adjust your land value to less than 0. This does not seem rational. If neighborhood values continue to drop, land could be mostly worthless, but will always maintain a certain percentage of value. Should the cost approach be substantially higher than the market approach, you know the homes there are selling for much less than "NATIONAL AVERAGES", which is what the cost approach gives you, with a subjective land valuation alongside. In the case where land would have to be adjusted to 0, to match the market approach, the market is obviously in severe distress, and the two approaches should differ on the report, with a comment explaining how the market here falls below national trends, and established benchmarks for typicall building pricing. Investors beware!
It seems like you are still in appraisal 101. Do you need me to spell it out any more clearly?