Curious to learn - what is a realistic life expectancy for a garage flush mounted fluorescent light fixture?

Discovered to my dismay that all three fixtures were dim and flickering. The house itself is 40 years old but the fixtures appear to be 10-20 years old based on the press fit connectors used. Garage is neither heated or cooled. Temps here can be teens in the winter to over 100 in the summer.

When I went to test whether the tubes were good I discovered a tombstone very stiff. So stiff I was afraid I might break the tube trying to put it in.. Even the original tube balked at going back in. In the process I also managed to snap a mounting tab from the tombstone. Do these become brittle over time?

Original intent was to put in new tubes if that was all that was needed. If ballasts were bad I considered reconfiguring for led's. But if the tombstones need to be replaced it seems maybe replacing the entire fixture with one designed for led's would make sense.

With that final thought in mind. Assuming a standard junction box is hidden by the existing fixture would a square or round flush mount ceiling light be a viable alternative? Say something like this?
http://www.homedepot.com/p/Commercia...3141/206589815