The rule isn't that spending bills can pass with a simple majority. The rule is that up to two bills a year can be passed via budget reconciliation. This is limited to one spending bill and one revenue bill or to one bill regarding both spending and revenue.
This tax reform was their one revenue bill this year. They could still do one spending bill.
The problem that was originally being addressed was that they were having difficulty passing one budget a year with a supermajority requirement. So they made it possible to pass one or two bills with a simple majority under certain limited circumstances. Over time, they've gotten better at exploiting those limited circumstances.