It’s an interesting thing to think about, but… like others have pointed out, given the progression from 4* to 5* in terms of rarity and needed ascension materials/cost, a 6* would really take an eternity. As a very active player, but without spending money on 4* ascension mats, it probably takes me roughly 5-6 months to get a set of mats for a rainbow 5* team (mileage varies as per the usual RNG being RNG)… much longer than getting mats for 4*. @Shallab’s guess of 2 years for a 6* doesn’t sound too far fetched. And if there’s a change of around 2% at the summons for a 5*, what would it be for a 6*…? Must be approaching something close to 0%… 
I think all things considered, such a major new addition would just throw off the existing balance way too much.
And in my experience, I’m nowhere near any sort of “end game” after about 14 months of playing. Just in recent months I’ve been able to level up a decent set of 5* - but still no yellow. And like with 4*, having your first team of maxed ones is great and a milestone, but after that there is use for many more heroes to deepen your bench.
All these things considered, I’ve mentioned this before, I think SG has achieved a great balancing act for this all to work, and allow for continuous progression for this long, and still really no end in sight. And for this to work with such a wide range of types of players, from entirely F2P to very big spenders, highly active and casual, and everything in between. My hat is still off to them.
