No.

After 5 years the first years salaries are considered to be a "finished" R&D project, which means there's nothing left to amortize.

The only thing like that is that if, after a few years, you fire all your devs (or at least, all devs working on adding new features and building new products), then you can keep amortizing the last 4 years of salaries for your now-fired workers over the next few years, which reduces your tax base. But I mean, by that point, who cares? Any software business that has fired all its devs is over.