Have a closer look at the players that play solos/melodies exclusively with fingers. Mark Knopfler and Derek Trucks would be good examples.
They both alternate between thumb and index a lot with other fingers joining in when handy.
Mark Knopfler in particular has an interesting technique where he the alternate plucks are more like a snaps where the string sort of gets passed back and forth between thumb and index finger. Have a look at him playing the fast bit of solo from "Sultans of Swing" to see what I mean.
Obviously speed is relative, and it's hard to build up speed using fingers only right up to what's possible with a pick but for some players out there the advantages of more control over the tone, color and articulation will prevail.
Also bear in mind that even though playing a fast notes on a single string might be harder with fingers, many other things are easier. Like changing string really fast, skipping strings ect. so it could be argued that some licks can be actually played faster with only fingers.
For example the way Mark Knopfler plays these little arpeggios - that has a unique sound to it and even though it could be approximated by someone with a very good sweep picking technique, there's not way to make it sound a little "portato" so that there are little pauses between each note.