Should I keep the first generation iPhone?

Not exactly sure if this'll become an antique one day.

It doesn't take up much space and you can't get anything of real value for it currently. Place it in a sealed zip lock baggie with some desiccant to protect it against moisture and put it in a box or drawer for safe keeping.

(If you have all of the original literature or packaging, save that as well. The more you have, the better.)

Absolutely, its worth nothing today so I would keep the box and manual also if you have it. Put it in your lockbox at the bank and forget about it for 20 years, seriously that will be worth soemthing some day.

Up to you. Yes it would be antique one day.

"Not exactly sure if this'll become an antique one day"

A thin calculators kept as valued antiques? How about old tube-TV's? What about Walkman cassette players, Palm Pilots, 486 Computers or the original iPods?

Machines no using solid state electronics can become antiques because they can be restored - if not to function, at least to see how their inner workings made it work

That's not true of solid state electronics - people aren't going to look at a printed circuit board or some transistors (assuming they're even visible) the same way.

As a rule, gadgets don't preserve that well and if they don't work, there's no other reason to keep them.