You've had lots of good discussions and good replies here, but it's still not clear if you're on board with this now(?).

I'll have crack but taking a wider view.

Newton came up with these laws to try to explain observations he'd made of the world around him.

He came up with words like mass, force, acceleration, momentum and then came up with some mathematical relations between them. These 'laws' allowed him to make predictions about the universe that were accurate.

Given that they were accurate, they were also useful! Hence they've been widely adopted.

None of us created the universe, none of us have a perfect understanding of how it works. Whatever is causing our universe to tick forward might not be using Newtonian dynamics at all.

There is no experiment we can do to prove that mass or forces exist. We can make observations that are consistent with Newtonian dynamics, but that doesn't make it universal.

In fact we know for a fact at astronomical length scales it isn't, because Newtonian dynamics makes incorrect predictions about the momvment of stars etc. (hello general relativity!).

We do, however, know these laws are often very useful at day-to-day length-scales.

So you can have your own theory that an object in motion tends to slow down, or requires something 'a force' to keep it in motion. But if you do, then you need to explain why satellites don't slow down in space and drop out of the sky. You need to explain why when I'm in my car on the motor-way, I don't feel the seat pressing into my back when I'm at constant speed.

On the flip side, your contention with Newtonian dynamics is that you have observed that objects in motion tend to slow down. Newtonian dynamics has an answer to that - in all these cases, there is an external force acting on the objects slowing them e.g. friction or wind resistance.

For me Newtonian dynamics, seems to provide a more complete picture of the world and does a better job explaining my observations. So, for now, I'm going to stick with using those rules to make predictions in my day-to-day.

Of course, it's a bit unintuitive this idea that objects at motion continue at motion... that's why Newton is widely considered to be a really really smart guy. He managed to get his head around that idea and ended up with a theory that made so many accurate predictions.