[MartinSE]

The fact that there was so much change from then to now is why they included the ability to amend the constitution - foresight.

Sadly at this point, amending the constitution is almost impossible - at least expecting the politicians to do it. So, if there is something we feel needs to be updated WE have to do it ourselves which is also an option.

So, what did they mean by "arms", I firmly believe they meant arms sufficient to protect the government from loyalists. And the reason they chose that route was because they could not afford (and did not want) a standing army. That too has changed. So, it could be argued, if that was the primary reason, that the justification no longer exists.