"there are lots of small bike shops in the southside, and Evans arrival in close proximity might kill some of them off"
It's true, but -
It's not quite the same as what happens to small food retailers if a Tesco (etc.) opens up.
I'm sure the Evans will be bigger than some other shops, but more 'metro' than 'superstore'.
They are not generally 'discounters'. They will have some brands that are available in other shops, but are unlikely to want to start a price war. They will probably have some brands that are not currently available in other Edinburgh shops.
Generally Evans has grown to where it is by a mix of marketing and service. It certainly won't be a case of 'we've opened in Edinburgh and expect all cyclists to use our shop and no other'.
Bicycle users are both fickle and loyal.
In an internet age price/cost is a factor, though it's only a couple of days since the "It pays to shop locally" thread was started on here.
In retailing it's generally thought that 'clusters' are beneficial. Now Tollcross/Bruntsfield/Marchmont is clearly the cycle shop cluster for Edinburgh.
IF any shops close it will be because they are not giving customers what they want - price/choice/ambience/service/etc.
The arrival of Evans will not be a zero sum game with the same number of people spending the same amount of money in n+1 shops.
Evans will mean more people riding more bikes, which is obviously good for Edinburgh.