The interesting thing is how easily you can 'slide' into a full rewrite, when your old code is aging legacy and everybody wants to get rid of it. I've participated and seen it happen many times. It has never worked particularly well.
In my experience, what you mostly underestimate is the amount of hidden features lurking inside a mature product, which all need to be rewritten in the new implementation. The stuff you can easily estimate is 20% of the work (main features), and 80% is all the small stuff you afterwards realize you also need.
In my experience, what you mostly underestimate is the amount of hidden features lurking inside a mature product, which all need to be rewritten in the new implementation. The stuff you can easily estimate is 20% of the work (main features), and 80% is all the small stuff you afterwards realize you also need.