Anyway, what matters most is the current trend/trajectory. The alt market doesn't look like it's seen capitulation yet and it's about to pass 70%. I don't see why we couldn't reach 80%.
The Bitcoin dominance declined from 85% to 40% in less than a year, mostly as a result of the ICO boom. If you remember, Ethereum gave a return of more than 100x for that year, since it had almost complete monopoly among the ICO platforms. Also, there were new altcoins coming in to the market, such as Cardano (which was predicted by many to replace Bitcoin as the no.1 cryptocurrency). The ICO craze is over and ideally we should go back to the 85% level that we were having before the ICO bull run. IMO, it is the ongoing IEOs that is giving some sort of respectability to the altcoin market. Without them, the altcoins would be dead as a dodo by now.
50% to 70% took only 4 months. We still have 4 months remaining so far in this year. I would say that 80% or even 85% is possible, if there are no surprises. None of the altcoins are able to hold on to support levels. Take Ethereum for example. It was unable to hold on to 0.018, after missing several support levels earlier. Litecoin is not doing that good either. A lot of hype was there for the block reward halving of LTC. But it crashed heavily after the halving and now it is trading at 0.0065.