Every day, new theories are being proposed. Serious ones. Serious enough to get published in science journals. Thousands every year.
Most of them are debunked within a few months.
Very few survive more than five years (less than 1%).
A theory is nothing more than a proposed explanation for something that we cannot go and check in person. It must be based on observations and on known processes.
The main value of a theory is its ability to make predictions (especially about things that are not known at the time of publication). Therefore, people are far more interested in the usefulness of a theory than on its actual "truth".
For example, we have known for a hundred years now, that Newton's theory of gravity, along with the model that comes with it, is "false". Newton thought that gravity was an instantaneous effect and that its scale did not change. We now know this to be "false".
However, for everyday life, here on Earth (and in the entire Solar system), the difference you'd get by using it instead of the more precise Einstein gravity (part of General relativity), is so small that we don't bother.
Einstein's model uses mathematical objects (and operations) such as "tensors". Very few people can tackle them and the calculations are long and tedious. Newton's calculation are easy enough to be high-school material. The "one-thousandth of one percent" difference does not justify the effort of going to the "better" theory.
We still use Newton's model (and the principles behind his theory) when sending probes to other planets within the Solar system. It is still "useful", even though it is not true.
Steady State (versus Big Bang)