Been running one or another 870 for about 30 years now, still got the first one I ever had (12 ga.) and it still works just fine. My wife and I each have a 20 ga. too, if anything they are even handier.
BTW, some 20 ga. 870s were built on full size receivers- if you want a 'true' 20 ga. be sure it's on a small frame. Full size receiver 20 ga. guns have serial numbers ending in X or N, small frame guns have serial numbers ending in K or U (per Jerry Kuhnhausen, _The Remington Shotguns, M870 and M1100/M11-87: A Shop Manual_, McCall, ID, VSP, 1992. P. 16).
Stay safe,
lpl/nc