Does this boiler need replacing?

I have an oil fired dry box boiler that is about 20 years old. I am told it is builder grade and that this style usually lasts only 10-15 years max. The bottom inside fibergl(?) is cracked and the unit is apparently hotter than normal on the exterior although it operates fine. Two people have quoted me new boiler setups, a third party who used to maintain it says I don't need to worry about replacing it until it fails. Thoughts? What would the failure entail?