Irrelevant what the CCFL specs are - the OP question was if it was detrimental to leave the screen on at high brightness. I have some empirical evidence that it is.
Moreover, I have a lot of experience of CCFL technology and how the claims of the lifetime and luminous output is often not met in real-world usage (across a sampling of 30 bulbs, a number of the ones that I have tested failed after 1000 hrs - even from high quality manufacturers)
The LED 50000hr spec requires a certain set of conditions, and more likely than not this might be at a nominal drive current that is lower than the maximum current used when the display is at full brightness
"The solid blue curve denotes the typical lifetime derived from LED manufacturers data. The shaded blue area on both sides of this curve indicates the performance deviation over different LEDs. This deviation might be attributed to LED flux binning7, batch differences, differences in aging conditions and so on."
Just like the MTBF figures that are quoted by hard drive manufacturers - many are based on (standardised) calculations - not real world "FIT" data. I know because I've done MTBF calculations for industrial products - they have little correlation to real-world failures, they are often mainly necessary for the marketing department.
Now, as for the OP, I think that if you will use the computer for 2 years then I would not worry too much about degradation, it will be fairly minimal. After another few years then you probably will notice it, but by then you're likely to have a lot of other issues (like image retention due to ionic contamination, but that is the subject of another lecture