*nod* *nod*. One big fat program-process is so DOS-era-ish. Many system daemons, e.g. cups, postfix, other MTAs, split up in some form, either for reasons of pluggability, better scheduling, selective restart, or in fact debuggability. In fact, even desktop environments do that. XFCE, KDE, even icewm, is not a single process.
I recall that 5-7 years ago, smbd subprocesses could go into an infinite loop ("Transport endpoint not connected" over and over and visibility in top(1)) if a Windows client suddenly disconnected in some form. Were samba, or even just smbd, a single process - it would have been hard to kill the thread. Were samba/smbd just one thread - it would have taken down the entire fileserving.