you dont need unreasonable numbers of armor to be useful. and you can get enough armor to be invincible on any class. its not limited to juggernots and guardians. you just need to know how to do it

my favorite way to get armor is using a unique jewel called grand spectrum - https://pathofexile.gamepedia.com/Grand_Spectrum_(Crimson_Jewel) . it gives you 200 armor per grand spectrum so it scales like exponentially. one of them is just 200 armor, but 2 of them is actually 800 armor because each one gives 400 and you have 2. and 3 of them is 1800 armor. 1800 armor makes you essentially invincible to physical damage. you can stand in groups of monsters and go afk without dying. its crazy good.

you can get 3 jewel sockets on any class pretty easily. scion can get them as early as level 10, but other classes might take a bit longer to get all 3. i always go for 3 grand spectrums whenever i make a new character. armor is just incredibly broken. id rather be invincible to physical damage instead of having 40% dodge chance