Crossing the floor should absolutely trigger a by-election

Parties do not get elected. When a human being gets elected to the office, they have sovereignty over that office. If they want to associate with one, two, a dozen parties, one one day and another the next, that's irrelevant to their holding office.

Parties do not have moral agency. Individual human beings do. Human beings have to be accountable, not mindless systems. Whips already have too much power. When members of government decide to defy their party rather than handing over the power of their office to an un-elected, unaccountable entity, it's usually a very good thing for governance; and when it's not, it's usually endurable.