Why should I switch to formula?!

My son will be five months old in a week. We have struggled with breastfeeding for various reasons, but I have managed to provide him only breastmilk so far. He has been sick for over a month now (since he started daycare) with various things. Now, more than one person is telling me that I need to switch him to formula to "help him recover". I see no logic behind this - or am I being overly sensitive because I have - and still am - fighting so hard to give him all the nutrition he needs and get him healthy? The doctors said cut out dairy, which I don't think is an issue but I am certainly going to give it a shot - but formula? Why? I do not have anything against formula or those who need/choose to use it, but this is illness, not a milk problem. My son has always been a big baby, and hasn't gained a ton in the last month due to sickness, but I produce plenty of milk and only see the positives that come from breastmilk while he is sick. Also, when asked how long I plan to breastfeed, I reply "as long as I can" and am told I need to wean before one year?! Can someone help me see the other side of this?