Screen has gone dull

I have a ThinkPad T20 (win98se) with an LCD screen that has gone dull (it has been flickering for a while, then yesterday it flickered non-stop and turned dull/dark) I can still use the laptop though the screen now is quite hard to read etc, since it is so dark. I hooked up an external monitor which interestingly enough only works as a mirror while I am not logged on as a user profile.
I don’t see the point in repairing it, since it’s 6 years old, I was just wondering how long you think the laptop will last now?, since I don’t mind using the external monitor.

I think that the laptop will work until anything else fails. Most likely the LCD screen has gone dull because of a failed FL inverter board. Neither a failed FL inverter board nor a failed LCD screen can prevent the laptop from working properly with an external monitor. The failed backlight will not harm the laptop itself. So, you definitely can use it with the external monitor.