PT vs hemlock for garden stair treads
Pressure treated will last significantly longer. Hemlock performs about the same as Southern Pine and its above ground life is significantly less than treated pine’s in ground life. In ground, you can’t expect untreated wood to last more than 3-5 years.

In ground, I think you can reasonably expect pressure treated to last six times longer (even more if you treat with an additional product like copper naphthalate). Given that plus installation costs, pressure treated is clearly the choice.

As far as other options, I think concrete or blocks would be the only other cost effective options.

References

https://www.fs.fed.us/t-d/bridges/documents/tdbp/decayres.pdf
https://bct.eco.umass.edu/publications/articles/wood-myths-facts-and-fictions-about-wood/