The unknown is what "success" really looks like, not only in a near-term sense but in the long-term. Russia may well control Mariupol, and all signs available to a person like myself half a world away point to that very much being the case, but what's to say that Ukraine will stop counter-offensives of this city/region? For all we know, Ukraine may consider this Ukrainian territory (regardless of what Russian's may believe) and continue the fight until it's back in their hands. If foreign nations continue supplying Ukraine with the finances, aid, equipment, training and intelligence (and whatever else may be required) to regain control of all Ukrainian territory, how long can/will Russia go on?3rd Mariupol' has basically fallen. Regardless of everything else, and the slow speed notwithstanding (slow compared to what though? major urban battles take time) Russia has succeeded in taking a large-ish city with many well armed defenders using a relatively small force, and relying heavily on rebel irregulars and even (rebel) reservists. Amidst the general failures of this war it is a success and it's important to look at it and see it for what it is. A half-destroyed city, most of its inhabitants fleeing or dead, 6 weeks of heavy fighting - a success.
Perhaps far-fetched, but what's to say that Ukraine stops there and doesn't continue on with Crimea? It is, after all, internationally recognised as part of Ukraine still.
Interestingly today I came across this video that, from an economic perspective, actually puts this scenario forward (The Price of War - Can Russia afford a long conflict):
Whether it's sound or not I don't know, my knowledge of economics is terrible, but I'm sure for those more economically knowledgeable it does bring up some interesting points of discussion. The short version being: the longer this drags on, the greater this advantages Ukraine - assuming foreign support remains strong.