And the issue isn't whether they'll take and hold a charge or not. The issue is chemical breakdown which leads to both the inability to take a charge and increased internal resistance. NiCds can cycle fine but not be able to provide enough amps to run your system.
And yes, absolutely NiCd's can last for 6 years and be fine for powering a low demand plane. But the average time of failure is getting very close at that age. I think of it this way. If I replace a $40 battery every 3 years, I've spent $13.33 a year. If I stretch that out to 5 years, increasing my chances of a catastrophic failure significantly, I've spent $8 a year. I don't think that $5 savings is worth the risk.