If you don't own bitcoin your chances of getting in early on any market-elected successor are roughly nil.

Firstly, alts are uniformly traded against BTC in the overwhelming bulk of their exchange until they gain at least a multiple of the prominence of LTC.  E.g. buying NXT with USD was impossible at first, infeasible for a long time, and now is merely very very difficult and impractical in the vast majority of cases.  (I do not consider NXT to be a candidate successor.  Firstly, it is tainted as a pre-mine scam, and being born in iniquity can never rise to the angelic sphere.  Secondly, it is extremely vulnerable to DDOS and largely untested in its security.  Thirdly it is one of many possible candidates, some several of which are designed to improve upon it, and since it lacks a legacy momentum, it is very vulnerable to a competitor -- unlike bitcoin, which has established billions of USD in momentum, motivated aligned interests &c.)

Secondly, any successor is likely to bootstrap its network on top of the blockchain network.  To do otherwise dooms you to scamcoinville for a very long time.  Why would you do that if you actually had a long-term plan?  Seek to handicap yourself from the inception by always competing with an 800-lb gorilla?  The primary reason to seize independence from the blockchain is to make a land-grab in scamcoinland.  The ceiling is quite low.  Not impenetrable, but very low.

For these two reasons I think a superior crypto is more than likely to add value to bitcoin, rather than detract value, and owning bitcoin now is the best way to situate yourself to participate in any future successor.