I agree with the OP, the bond should be scaled. I've been playing since release and I had a level 65 adventuring fellow before there was even a 70 cap so I'm unbiased and haven't had anything handed to me.
Now let's look at this both from play style point of view and a lore point of view.
Playstyle, a level 50 adventuring fellow is going to be of no use to a level 99 player, if the fellow reaches the level 50 cap then it's stupid to have to wait a week maybe 2 to then cap your fellow bond. Would you be happy with that if, as a player, when you reached level 50 you then had to wait a week or more to reach level 55 then another week to 60 and so on? There is no added challenge in summoning an npc but there is also just as little reason to summon them at 99 because they can't do anything and they are gaining nothing. So in the end it's just a waste of time, the challenge of levelling an npc should not be in the fact that they hit a wall and have to wait to progress, it should be in actual game content e.g. The limit break quests.
Lore wise, <<character> uses a signal pearl>. <fellow> "Hi! I'm ready to go, what are we doing?". <character> "oh nothing, I just called you to say hi". <fellow> "oh... Couldn't you have done that at a rendezvous point?". <character> "nah, I did that earlier". <fellow> "um... Ok, so.. Why did you feel the need to say hi?" <character> "to make you like me". <fellow> "you called me, your level 50 friend, out here, to an area level 50's dare not tread, to say hi... In an attempt to make me like you...?". <character> "yep! And now I need you to leave, cause I don't want you in my party lowering my exp gain." <fellow> "**** you man, you're an ass...". <character> "watch out for the true sight death scorpions on your way home!" -<fellow> leaves-. <character> "...". <<character> uses a tactics pearl>...