Extrapolating GLM coefficients for year a product was sold into future years?
If you suspect your response is linear with year, then put year in as a numeric term in your model rather than a categorical.

Extrapolation is then perfectly valid based on the usual assumptions of the GLM family. Make sure you correctly get the errors on your extrapolated estimates.

Just extrapolating the parameters from a categorical variable is wrong for a number of reasons. The first one I can think of is that there may be more observations in some years than others, so any linear extrapolation needs to weight those year's estimates more. Just eyeballing a line - or even fitting a line to the coefficients - won't do this.