"And the technology can only improve, get cheaper, and be more available"
Obviously. Most nations use laser-guided munitions these days, just like they adopted proximity-fused artillery shells. Often the hard part of engineering is proving something can be done. Once proven, it's quite a bit easier for others to duplicate. Or buy on the open market.
"What happens when anyone can get hold of a weapon that has a sure kill at such a range that you can't spot the attacker in advance?"
Regarding "sure kill," I think you're overrating laser-guided munitions. They do miss, they give a lot of warning when you start lasing the target, and they are subject to countermeasures. Laser-guided munitions are hardly new threats.
As for not spotting the attacker in advance, militaries have been dealing with that problem for a long time. Snipers were firing from unseen positions centuries ago and the 20th Century gave a lot of examples on how to function when over-the-horizon artillery started dropping shells on you.
"For a start no US president will be able to appear in public ever again. "
Supersonic anti-tank guided missiles with ranges of miles did not drive VIPs into hiding, even though most VIP teams have no defenses against them. No, the Popemobile and Presidential Limo don't have active defenses or reactive armor to shrug off an ATGM. A more-accurate small caliber weapon is likewise not going to drive anyone into hiding especially considering the preferred range of most assassins.
No VIP security detail is perfect. They reduce risks and provide a deterrent. When someone - or some larger organization - puts effort into defeating that security, you will run into situations that overmatch the security. Heck, lately the Secret Service isn't even stopping threats it should be able to handle. Reagan was shot by someone standing among his security detail, and a knife-wielding maniac just jumped the fence and ran through the White House.