Why hasn't Xbox started a reservation system for new consoles for their long time Xbox live...

I have two xbox one x systems and the second system in the den got the update over a month ago, but my main console hasn't as of yet. Why so long?

There hasn't been any policy changes. A always or periodic online check once an Xbox system has completed the initial set-up (which must be completed online) that the system must from time to time connect to Xbox Live, is untrue.

Once a system is set up, you should be able play any game, as long as the game in question has no mandatory online functionality, offline, with the console itself not connected to internet either via Wi-Fi or an Ethernet wired connection.

This offline function also applies to Xbox digital downloads. As long as the console was set to Home when connected to Xbox Live, all digital downloads - assuming they have single player functionality, will also happily play offline.

Appear offline is just your visibility on Xbox Live. To truly be 'offline', you must either select this in the network options of the console or disconnect your console from your own network.

Actually, even if there's no Xbox live account associated yet on that account, the system will not allow you to add it as an alias. All Microsoft related emails are reserved domain so you may need to use a 3rd party email. But if what you're trying is to
transfer your gamertag to a new email. Then doing it over an xbox 360 console will allow you to use a Microsoft email as long as it does not have any gamertags yet.

" I hadn't played in my Xbox One for quite a long time, but recently I received an Xbox Gold card so I thought "Why not" and decided to turn it back on and activate it."

Depending on how long ago the console was turned on there have been many updates to the General Availability
XBox System updates and they may need to be downloaded and applied before you can continue using the console.

As SavvyReigns mentioned, allow the updates to complete prior to starting a game.

I think that it makes sense for a new console from a new generation of systems to be coming out in holiday 2020, which would be 7 years since the start of the Xbox One generation. With mid-generation console editions being common now, it does mean that
the time in which the most recent console version will be the freshest console on the market will be shorter than before; that said, I still expect the Xbox One X to be a very relevant and powerful console for a long time into the next series of consoles.