When you're talking about a constellation of 24 satellites, a short operational life means you're constantly having to send up replacements. If each satellite only lasts about 18 months, you have to completely replenish the constellation every year and a half. IIRC, they launch Glonass satellites three at a time on Proton boosters. If that's correct, you need 4 boosters and 12 satellites a year to maintain the constellation. That's hardly an efficent way to operate.