The Doc4444 (google it) is the bible of air traffic control. It is the base for all international standards of separation, and in section 7.9.1 "Separation of landing aircraft and preceding landing and departing aircraft using the same runway" we read:
Except as provided in 7.10 and Chapter 5, Section 5.8, a
landing aircraft will not normally be permitted to cross the
runway threshold on its final approach until the preceding
departing aircraft has crossed the end of the runway-in-use, or
has started a turn, or until all preceding landing aircraft are
clear of the runway-in-use.
Now there are some national deviations from this in for example the US, but the basic principle is for a controller to NOT put him/her self in a monitoring situation.
If the controller were to clear nb2 for landing while nb1 is still on the runway, and something happens to nb1 that unables nb1 from vacating the runway, the controller is in a hurry to restore separation by cancelling the landing clearance to nb2.
If the controller instead follow the doc4444 and don't clear nb2 for landing, and something happens to nb1, the controller can do whatever he needs to do to help nb1 (send out fire trucks or whatever the problem demands), and IF the controller forgets to instruct nb2 to go around, that should be the default behavior of nb2 anyways since he never got a clearance to land (if no landing clearance is received when reaching the "missed approach point" the pilot will initiate a go-around). That way, separation was maintained the whole time and needed not to be restored.
Controllers are trained to always take risk into consideration, and eliminating it whenever possible. In the case of your cessna, depending on the experience of the pilot (many small planes are flying for schooling purposes) he may not touch down in the touchdown zone, especially if there is a crosswind. Students sometimes make a few bounces before settling on the runway etc.
Your reasoning saying that "there would be more than enough space" could also be used to argue for allowing a departure with a smaller aircraft when a larger one is way down the other end of the runway, "the little one would be airborne long before the big one anyways", right? But a controller will think "IF the little one has an engine failure (yeah, the small ones usually only have one engine...) on initial climb, he will glide back down on the runway. So we don't allow any type of obstacle on the runway in front of a departure either, no matter the size =)