No.. I wouldn't want the sicknesses to last longer unless other changes were made. Right now to me they are mostly just irritating when they happen so I'm glad they are over pretty quick for the most part.
What I'd like is for them to add in ways that we could always have "well" sims.. like booster shots once a year or the beginning of every season or even a trait(s) An instant cure option would be good also... we already have medicine and snake oil (GTW).
Then we could have longer sick times and make sickly type sims if we want... and for those of us that don't we have gameplay/options to make sure we don't.