I think the hard/soft counter debate is meaningless for rts design, they are both legit choices depending on how those units are designed. first of all, a game can have both soft and hard counters, look at 0ad: sword cav is a soft counter for skirmishers, but siege towers are hard counters for archers, rams are for siege towers, and swordsmen are for rams...
I think that for some units it's good to only have soft counters. if it feels good for them, why not? take medium/heavy infantry in "realistic" rts games like total war: they are the bulk of most armies, and it's only right that, however you are going to face them, it will either take a lot of time or quite some losses to take them down. of course hard counters are also good if they are well designed, and can be very fun.
soft counters do lean towards snowballing, but some hard counters are enough to offset that I believe.