It will take at least several seconds for your PARALYSE-VENOM-CLEAVE combination. During this time, your target (who you’ve somehow caught alone - rich people tend not to be alone) can yell to their heart’s content, summoning all sorts of people to their aid.
You then have to actually loot the dying body - which takes more precious time whilst your target continues to yell for help. Also, rich people tend not to carry all their personal wealth with them, so you’d only get away with a fraction.
Then, let’s assume you get away without being caught. Even if no-one rescues the dying person in time, someone will cast Speak With Dead to find out everything they remember about you. Now you’re being hunted, and word will spread quickly. People will start going around in groups so as not to be mugged.
And, at the end of the day… it’s a pretty boring story. “Rich person was killed in an alley, had stuff nicked”. You’ve maybe gained a few thrones at max, and are very likely to be caught.
In your “group of people” scenario - now the whole group of you have to escape the scene without anyone catching any of you, or seeing who you are. Once one person is caught, it’s often very easy to find everyone linked to them.