I have 6 kinds of tyme covering a hill with the some shrubs intersperced on the hill. I find the tyme gets a bit leggy and needs a good haircut every so often and not the best for barefeet but certainly handles foot traffic. I live in Zone 6 but I did use it in Calgary grew slower but survived the winters. The weeds seem to always find their way in thats life. Irish moss is much more delicate .Scotch moss on the other hand pretty tough and grow in zone 4 -8 quite well and feels not bad on bare feet.

Irish moss is exactly the same plant as Scotch moss except for coloring. Exact same care and growing conditions required. But not very xeric :-) Pretty much the same watering requirements as for a traditional lawn.

If the grass is being trampled out no alternative low carpeting plant is going to hold up better. And grass has the advantage of being regularly mowed, as far as weeds are concerned - with the mowing these can at least be kept below mower height. Whereas anything like thyme - that does not lend itself to mowing - has to be hand weeded, as mentioned above. And when a planting consists of such very low growing subjects lots of weeds will come, unless kind of ground cover chosen and site conditions are in perfect balance.

For instance I was once charged with weeding an established woolly thyme planting with a dinner fork. The issue was that - although the drought adapted thyme had done well - the soil on the site was moist enough to support numbers of tiny rush plants that were poking through the thyme carpet.

I could have spent the rest of my life going back on a routine basis and popping these rush plants out of that planting.