If your doc has the results of a recent colonoscopy and he has your word about
your current condition, what examination would you have him do? He pretty much has all of the information he needs right in front of him to determine what should be done next. Are you in a flare that is hard to control right now? Have you been flaring for a long time? Entocort and Pentasa are kind of mild drugs, so your doctor probably believes you need something stronger. It really depends on the state of your health right now- which you didn't tell us what that is.
I was on remicade for 6 & 1/2 years the last 9 months of which it had lost effectiveness - just took me a while to figure out. I highly doubt that you will be on remicade for the rest of your life because unfortunately, the body will build up antibodies to the meds and it will stop working. Then you will move on to another biologic hoping that it works. Or at least that is how it usually goes.
Is your GI receptive to your questions? You should make a list of questions for him and ask. I think it will help you feel better to find out why he's making the decisions that he is AND you can therefore make a better decision as to whether or not you want to go that route.
P.S. - for what it's worth - I was severely flaring when I started remicade and it worked WONDERS for me. So, because of my positive experience I would always tell anyone that I think it's worth a try.