So in the future, fiat currency will become digital only and cash will completely disappear. Credit cards will also disappear. People will be implanted with chips that contain all their information and they'll need the chips when buying or selling goods
I have many questions:
How would bitcoin coexist in the future with this method of payment?
The same way as it is today. Maybe will be a mobile wallet where someone can have many cryptocurrencies and fiat as well.
Will it get accepted as a second currency?
There is none could answer 'yes' or 'no'. But, who really cares? The point is to be a currency and if is possible to be accepted in many places in the world. Second or third; only the conversion rate has meaning.
What about other crypto currencies or bitcoin forks?
About other cryptos the same as it is today. The more strong ones, with a big user base will achieve to survive, some of them will be used as a medium of payment and the rest will die. As far as it concerns the possible bitcoin HF there are so many threads in the forum where you can be informed.