Another detail that is not documented about the legacy urlbar: we distinguish results coming from the network from local results, because they have different characteristics.
For example, we put search suggestions before historical results, this means that every time the user types a new char, if remote results are slower to arrive, we'd end up adding historical results and then they would be pushed down by search suggestions, and this causes continuous flicker while typing.
What the old urlbar does is replacing same-type results, and once a search is done it removes the "stale" results that don't apply anymore. This prevents flicker of results, because before removing search suggestions the view waits for new ones to arrive (and often they may be the same already there).
This is implemented in UnifiedComplete in the legacy implementation, but in the Quantum Bar we can't reuse that, because the new API inserts new results one by one and lets the sorting to the muxer. Though, the view can likely do something like this using the result type/source.
This flicker prevention is probably more important than any resource saved by reusing elements.