First off, no, a full rewrite is not only not necessary, but probably the worst possible approach. Do a piece at a time. You will eventually have re-written all the code, but do not ever fall into the trap of a "full re-write". It doesn't work.
But before you re-write once line of code - get some testing in place. Or, a lot of testing. If you have end-to-end tests that run through every feature that is currently used by your customer base, then you have a baseline to safely make changes. You can delete code as long as the tests pass. You can change code as long as the tests pass.
Once you are at that point, start picking off pieces to modernize and improve.
Also, respect the team. Maybe they aren't doing what you would, but they are keeping this beast alive, and probably have invaluable knowledge of how to do so. Don't come in pushing for change... come in embracing that this beast of a codebase makes 20 million a year. So talk about how the team can improve it, and modernize their skills at the same time.
Because if you walk in, saying, "This all sucks, and so do you, lets throw it out", do you really have to wonder why you are hitting resistance?
But before you re-write once line of code - get some testing in place. Or, a lot of testing. If you have end-to-end tests that run through every feature that is currently used by your customer base, then you have a baseline to safely make changes. You can delete code as long as the tests pass. You can change code as long as the tests pass.
Once you are at that point, start picking off pieces to modernize and improve.
Also, respect the team. Maybe they aren't doing what you would, but they are keeping this beast alive, and probably have invaluable knowledge of how to do so. Don't come in pushing for change... come in embracing that this beast of a codebase makes 20 million a year. So talk about how the team can improve it, and modernize their skills at the same time.
Because if you walk in, saying, "This all sucks, and so do you, lets throw it out", do you really have to wonder why you are hitting resistance?