I've found that SLAs tend to have really poor life.
Batteries don't die. They are killed. Well looked after quality SLAs will last at least as long as the data sheet says they will.
Example, I have some Vision 55AH units here with a 10 year design life, and some CSB 17AH units with a 5 year design life. The Vision batteries are now into their 13th year and the CSB are into their 9th year. Both sets test out at rated capacity. I have 14 & 12 of each respectively. No early life failures, and a well looked after, temperature compensated float towards the lower end of the datasheet float voltage.
If you don't overheat them, don't overcharge or undercharge them and keep them within their datasheet cycle parameters, they'll last at least as long as they are rated to. On the other hand, I had some 17AH CSB batteries in an older APC UPS. Kept at about 35-40C and with an un-compensated float voltage right at the upper limits, and they were mostly dead in 3 years and totally dead in 5 years.
Like anything, people don't pay for quality and tend to treat batteries as a "set and forget" appliance. In those instances they won't last.