It's really hard to keep your supply up exclusively pumping and only feeding bottled breastmilk.
You'd have to express at least every 2 hours (or more) and for 15 minutes after the milk stopped flowing. Basically if you were exclusively expressing you need to stimulate your breasts (express) as much as the baby would feed if exc. BFing.
Most people find their supply starts to decrease the more they pump and the less they breastfeed (from the breast).
Also, personally I find the baby feeding a lot more pleasant than when on the pump. Breastfeeding, once you get over any initial issues, releases oxytocin.
I do however plan to express (and not bottlefeed). I always leaked loads with my daughter and never gave her a bottle. Most of the milk ended up in breastpads and down the sink as I didn't express it, it just leaked out especially when feeding (from the other breast). This time I'm planning to express and donate the milk. I've already spoken to someone local who's due in September and is wanting donor milk for her baby so will donate to her most probably.
But, I'll be expressing from one side whilst my baby feeds from the other side, as this maximises milk production & how much the pump gets out.
Exclusive pumping can work, but it takes a lot of effort and you might find talking to a La Leche League Leader/support group (antenatally) would help you prepare for that.