Yes, this kind of thing worries me too.  If someone did come up with a bitcoin-like technology (BTCv2) that was substantially better in some way then presumably more people would invest in it, more merchants would accept it etc.  This would clearly devalue the BTCv1 w.r.t other things (usd, eur, gold, land, whatever) because there would be the same supply but less demand.  But of course BTCv2 will come along at some point, that's just innovation.  Will it be backward compatible?  Can it be backward compatible?

As I see it, the only thing that could save BTCv1 in the long-run is the 'network-effect' of many people having already invested a lot of money in it.  The longer bitcoin goes without a competitor the more embedded it will become, hence less likely that it will be displaced by a non-backward-compatible BTCv2.  But how embedded it needs to be to not be replaced by better technology is anyone's guess.