For example, because of the way car and property crime are (not) investigated, a petty car criminal can reasonably expect to get away with their crime.
There are plenty of other crimes where a police authority would choose not to take dramatic action and save a £10m cost of policing.
The suggestion isn't Assange be allowed to walk. If he were able to be affordably apprehended, then that is reasonable. The problem is that this spending is on policing theatre, aimed at making clear (primarily to the US) that the UK takes Assange seriously as a threat to security.
This kind of performance wouldn't be put on, at this expense, if a random alleged-rapist took shelter in an embassy. The police would leave them to it, and act when they had intelligence that they'd slipped out.