I think you're misunderstanding me. I'm not saying multiclassing is broken and doesn't deserve the help. The PHB ranger and the paladin are the only classes I would say unambiguously gain from multiclassing. The PHB ranger is so bad it got a UA revision. Paladin is great, but it has so much synergy with bard and sorcerer that it feels silly not to go into those when the losses for doing so are very minimal. Every other class is giving up something substantial to multiclass.
I'm saying that you're removing one of the advantages of a single-classed character unintentionally. The fix I suggest is to give out some +1s to ability scores here and there and not change the rules of ASIs. This way you stay equitable with both single-classed and multiclassed characters. You make multiclassing more viable, and the single-classed characters get to keep their feat advantage.
Warlock 2/sorcerer X is not at all broken and is actually a trap. Delaying spell progression for Eldritch Blast shenanigans is a bad idea, and the build is promoted by people who obsessively calculate at-will DPR to the exclusion of all else as a measure of power. It's a build that shines up to level 6 and quickly falls off compared to the single-classed sorcerer. Even at level 17 when you have 9th level spells, going into warlock sacrifices extra 5th-7th level slots that could do a whole lot more in one turn than Eldritch Blast stuff can over an encounter. Every 9th level caster suffers a bit for multiclassing before 17.
Paladin 6/warlock X is better than warlock 2/paladin X. Paladin 2/sorcerer X is better than both in the long run, as is paladin 6/sorcerer X. The warlock versions start stronger because of the short rest slots, but the sorcerer versions pull ahead when accumulating spell slots and metamagic kick in. Paladin 2 or 6/bard X doesn't bring the raw power of the sorcerer build, but in the long run provides a lot of utility, better spellcasting, and more smiting power than the warlock builds.