Most boilers are repairable therefore the need for a new one can be eliminated but there are exceptions that can lead to boiler replacements .
1.The efficiency rating of the boiler , all modern boilers are at least A rated now.
2. Water leaks which that can also cause damage to the electrics.
3. A clogged main heat exchanger.
4. Poor water temperatures and restricted hot water flow rate.
5. Main parts need replacement i.e. PCB water manifold .
These are some concerns when considering a replacement