I am concerned that using very sightly worn Shimano SPD cleats and M520 pedals is too loose.

I rode about 3000mi on one pair of cleats, always and frequently unclipping with left foot. At about 2000-3000 miles the left foot barely held tight, would easily pop out at unexpected moments. Pedals are tightened to maximum. I replaced the cleats and immediately noticed a strong connection. However now with only 400mi on this new cleat, they are popping out much to easy, even with a very hard vertical pull (i.e. with no ankle twist.) I then tried a new M520 pedal with this 400mi worn cleat and it also could be popped out.

So it seems that after 400 city miles (i.e. lots of unclipping) that the cleat is worn to become near unusuable.

1. Is that normal to have to replace cleats after every 400mi or so?
2. Are there SPD cleats that wont wear as quick or hold better?
3. Is there a different clipless system that can be mounted to the two hold SPD pattern and is can be recessed into shoe?

Thanks,
Al