With many new cryptocurrencies sprouting in the net, is it possible that bitcoin can be overtaken as a top digital currency in the future? Is ethereum or other well known cryptocurrencies can be a threat to bitcoin dominance?
No, Bitcoin is all the needed. You can build virtually anything arounds Bitcoin.
Bitcoin is crap, Transactions overload problems, code can't be updated by Dev Team because the Chinese Mining Pools won't let them.
Price is being artificially inflated by it Centralized Miners.
(Does anyone believe that crap is really worth $700, sucker born ever minute) 
Wastes loads of electricity that will raise everyone electric bills.
The question is not When will another Crypto take over,
The question should be How has BTC hung on to the top spot this long?
(Sad reason, is most people don't really understand the technical issues, they will learn and it is going to cost them) 