Viability of argon-oxygen atmosphere
One may think that Argon, being an inert noble gas, causes no harm at all and can perfectly replace N2 in a mix with oxygen. After all, you've been breathing Argon your whole life, since approximately 1% of the air is Argon:



 



However, we are talking about a raise from 1% to 78%... and there are possible consequences here:


Argon is way denser than N2: Argon has a density of 1.784 g/L, while N2 has a density of 1.251 g/L (O2 has a density of 1.428 g/l). So, Argon is 40% denser than N2 (and also denser than O2 and denser than air). Without taking into account ventilation and other forces, there is a risk of argon not mixing properly with the incoming "air" in your lungs.
Argon is a narcotic: This may sound strange, but there are evidences that Argon has sedative properties. According to Nowrangi, Tang and Zhang (2014):


  Argon gas is considered a small noble gas element that has been applied in a number of fields. It has been generally classified as a
  nonreactive or inert gas providing a view that it does not contain
  any biologically active characteristics. In fact, argon has
  demonstrated characteristics such as narcosis [...] The mechanism in which argon displays its anesthetic ability has been suggested to be from the stimulation of γ-aminobutyric acid type-A receptors. (emphases mine)



As an additional information, there is an Argon-Oxygen mix, called Argox, which is used for decompression research only, not as a scuba-diving breathing mix. 

Also, this question from WorldBuilding has some interesting information (and some wrong information as well).

In conclusion: this Argon-Oxygen mix is not harmless.



Source: Nowrangi, D., Tang, J. and Zhang, J. (2014). Argon gas: a potential neuroprotectant and promising medical therapy. Medical Gas Research, 4(1), p.3.