The consequences of "there's some debris in a little-used MEO orbit for enough decades for it to take for somebody to be bothered to deal with it" are a little less drastic than "the world can't use satellites for 5 years" with the kinds of LEO crashes you'd be encouraging by replacing single satellites in higher orbits with dozens in lower. So even if the unit economics of replacing a geostationary satellite with a constellation large enough to maintain that continuous line of sight from a 500km orbit were acceptable, it would still be the exact opposite of a safety-enhancing move.