Relying on the whip's feature, minions which can lock on enemies for high-speed attacks are highly effective, while other summoning weapons seem no longer important.
If you mean only Sentries this is true, but that has little to nothing to do with whips at all. "other summoning weapons" are minions & whips and most of them are fine and great to use, so I don't understand. When it comes to unviable minions, this is also not a problem with whips, simply themselves being unviable.
The whip's feature narrows summoner's options somewhat.
The overwhelmingly opposite is true though. Summoner "gameplay" in 1.3 was either pretending to play the class by actually playing Mage/Ranger in disguise while minions did little to absolutely nothing in boss fights, or desperately flinging yourself close to bosses while praying the minions do something. It really boils down to that.
In 1.4, Pre-Mech alone has either Obsidian/Cracker/Sanguine which is a relatively "high range" glass cannon build, or Spider/Cooler SnapTap/Blade which requires mastery of whips stacking and sticking closer to bosses, but has higher survivability if going full Warding (damage percentages bonuses don't affect Blade staff builds nearly as much).
What about Wall of Flesh? Obsidian SnapTap for glass cannon WoF shredder, or the defensive Bee Armor strategy with a mix of Frogs & Imps boosted by the Sharpening Station interaction bug.
Summoner actually has a lot of options as far as playstyles goes, and I mean actual playstyles. Not just using a different weapon or armor set with a very similar playstyle. The way you approach boss fights as a whole is wildly different with varying Summoner builds.
The whips add a ton of options.
Shall the playstyle of whipping become universal, or do we need more whip buffing armors (after loosing the obsidian)?
Almost every optimized & efficient Summoner builds uses whips. But I'd love to have Tiki armor obtain Whip bonuses. Perhaps current Obsidian armor with higher defense?
The fact is, only a few players can use summoner to its full potential, by high techniques, and summoner is still not fun for most players. Whip-stacking seems to be a flaw, technically. I tried it a few times, then it me me feel like I'm taking advantage of bugs.
In a way you could say the same thing about Wavedashing in Super Smash Bros. Fact is, an option requiring a higher skill ceilling to max out its potential is not a bad thing at all, and it's actually a good thing because it adds depth to games if anything. Whip Stacking is not even remotely
required to play Summoner let alone efficiently kill bosses with it. Fun is also subjective. That said, how many people realistically played "Pure Summoner" in 1.3, let alone for a "casual" playthrough and not intended for a challenge, because playing 1.3 pure summoner was by itself a major handicap-like challenge?
Quite a few post-Plantera summon weapons have been ignored. You can call them useless, cuz mostly we just go from Sanguine Staff to Stardust staffs. This makes the summoner process monotonous.
This is again not about whips nor should whips be blamed for some minions simply being trash by themselves. Also, Raven Staff and Desert Tiger are plenty viable. Blade also exists.
Some whips designed to have special effects, some designed to gain speed, they are two ways of raising DPS. If we do need whip speed, we can get it from equipments.
This game does not need & can no longer handle adding and balancing "+35% melee speed" accessories and similar for a whip balance overhaul.
amd minions that don't work well with whips may also need to be improved, though it's not known whether simply adding stats will work.
There is essentially no minion in the game at all that doesn't significantly benefit from whips. The minions that appear to not work well with whips are simply very weak and unviable by themselves, as I keep saying.