lenses cut 15-20% of the light output of a fixture. plus they collect "critters".
Lightolier just cam out with an IC rated linear troffer.
T5 lamps are about 4 times as expensive and only 50% longer life. plus they have to run hotter. light levels **** in the cold.
T8 fixtures and larger...
lumen depreciation is a problem once lamps (light bulbs) reach their rated life. some lamps will continue to burn, but produce maybe 30% of their initial output. most large retailers relamp a store every 18 months to two years, 1 - because it's cheaper than have a guy run around changing lamps...