What's the appeal of traditional telcos, considering how there are many new providers such as Giga! and Circles?

Even if you want a discounted phone with traditional telcos , you'll save money by just getting a SIM only and using the money saved to buy the phone up front. I guess traditional telcos are selling you the phone on a installment (with interest ) basis which appeals to some people.

But then again with phones being as powerful as they are now (even for mid rangers), there is little reason why one should need to upgrade their phones once every two years

On the topic of reliability, I think both traditional telcos and SIM only telcos are equally good as SIM only telcos leverage on traditional telcos cable lines. For example, Circles using M1 cable lines. Hence, there should be minimal differences in terms of reliability.

With regards to appeal, I personally feel that the appeal of telcos would be their "heavily discounted" mobile set which allows the user to use the latest set every 2 years or even on a yearly basis. Also, even though monthly phone bills may seem exboritant, many customers probably leverage on company discounts etc. to offset their monthly bills.

If you are alright with using the same phone for many years (and if it is durable) then a SIM only plan would be suitable for you.