There are definitely two sides to every story, I just read a thread on here not too long ago about a mother having major difficulties with the very thing your OB mentioned, about your son possibly regressing once the baby is born. Anyway, I just wanted to point that out, it is obviously something that works for some and not for others and I guess it all depends on the child/children involved. This particular lady was having one heck of a time nursing her toddler son and her infant, it got to where the toddler son was drinking all the milk and leaving none for the infant and mom was not able to keep up enough of a supply for both. And the toddler son did NOT want to be weaned, he wanted breast milk all the more after the baby was born. The poor lady really had A LOT to deal with, with the milk supply problem and the toddler throwing tantrums cause he demanded breast milk plus she was fighting the flu. I may have to go see if I can find that thread again just to see out of curiosity if she was ever able to resolve the issue. I sure wouldn't want to be in her shoes, what a horrible ordeal!
It is often true that a toddler may suddenly wish to nurse more once the baby arrives. So, that is something to be prepared for. But, then toddlers often stop using the potty or doing other "big kid" things, too. There's no magic mirror that will show you how your toddler will respond to a new baby. For some, tandem nursing may ease the transition. For others, it may make it more difficult. Tandem nursing may not be for everyone, and your opinion may change one way or another as it gets closer for baby to arrive, or after the baby is born. Adventures in Tandem Nursing really describes all the different scenarios and situations involved: the good, the bad, and the ugly. It's okay to change your mind at any point and wean the older one. I always left that option open, and just took it one day at a time. (And now I'm nursing 2 through a third pregnancy! )
But, for a healthcare practitioner to make blanket statements that nursing at some point is not beneficial or even harmful or to tell ANY mother at ANY time that she SHOULD wean or X,Y, and Z will happen is just ignorant and wrong.