@coyote Japanese particles are actually function words, not inflectional endings. You can put a parenthetical phrase (surrounded with dashes) before a particle like を because it's a separate word, but you can't put one before an inflectional affix like the English plural -s. Most of them are more like English prepositions, though they come after rather than before, so you might call them postpositions.

A lot of folks say Japanese has a case system anyway, though, an analytic case system―that is, formed with separate words rather than inflectional endings

So you could say 私が is nominative and 私を is accusative because they bear the nominative and accusative case particles が and を

But they're still not really inflectional endings.

They seem similar, though, don't they? Most of the time in Japanese, particles are pronounced as part of the same accent phrase as the word before them. And they often indicate the grammatical role that that word has.

Someone else could probably make a much more rigorous and detailed argument to show that particles are actually separate words. I have some examples saved somewhere to show their distribution but I'm not sure where...

As for shifts to/from declensions or particles... That would be a good question for Linguistics.SE

There is a general trend among languages, though, where syntax turns into morphology over time

Anyway, I think you should feel free to post whatever you want on that topic :-)

But if your question is really about linguistics, or about languages in general and not just Japanese, I suggest going to Linguistics.SE instead of Japanese.SE

Especially lately we don't have a lot of users who focus on linguistics here, it seems.