Of course they were under no legal obligation to step down before the end of their six year term, why would they? it’s in the Constitution. Have you ever heard of a Senator step down now after their state switches to another party? The Senate was there to represent the states, it now only represents political parties and the corruption is now easier.
You’ve just made my point. What the Founders intent was that the Senators would vote the way they were directed to do so by the state legislatures. If the Senators refused to follow said instructions, they would do the gentlemanly thing and step down and allow the legislature (or Governor, if not in session) to choose a replacement who would. When the Senators stopped doing that early on and acted independently or in opposition to their state legislatures, it ceased to be what it was intended to be.
The only way to have kept it as the Founders intended was to have made it so that if a Senator went off the proverbial reservation with respect to his state, that a simple legislative majority vote could remove them from office with an immediate election triggered. That way, you’d at least have had close responsiveness to the state legislatures as elected representatives of the people.