Where does it say that modern people absolutely cannot become Heroic Spirits? I know it's difficult, yes, but impossible?
Q: If there are Heroic Spirits like Archer, who was summoned independent of time axis, is it possible for warriors from the future, who are loaded with modern weapons, to be summoned as Servants?
A: Although modern weapons are powerful, it is difficult for the users of modern weapons to be classified as Heroic Spirits. The advantage of modern weapons is "anyone can use them as long as they were trained", thus it is difficult for the user to become a Heroic Spirit,
whose definition is "the one and only". The users of modern weapons are faceless heroes. If there are indeed heroes who are loaded with modern weapons, the users
will not become Heroic Spirits, as they are merely "experts anyone can achieve through sufficient training".
The actual Heroic Spirits being worshiped will be the "greatest weapon of that era". However, for the weapon to become a Heroic Spirit in this case, it must harbour a soul.Taken directly from the Comptiq special issue Fate Dojo.
Yes, it only says "difficult", but it also goes out of it's way to point out that it's less the hero that is worshiped and more his weapon, he is just a faceless hero. A faceless hero will not get worshipped, for nothing he has is special, whereas Heroic Spirits are specifically the ones capable of the seemingly impossible and therefor worshiped and remembered by mankind. Whereas Counter Guardians like EMIYA just have to sign a contract that binds them to Alaya post-death.