Does anyone else believe syndicalism is the only viable path to socialism?

What I mean is that socialism is already hard to imagine, but it becomes easier the stronger the working class is organised.

I agree with this but from that it doesn't follow that syndicalism is the only viable path to socialism.

The old arguments against syndicalism are still true. The battle for better conditions within capitalism is not the same thing as the battle against capitalism. Unions need to be big to be successful. Unions can be set up with revolutionary programmes but in times of the majority not holding revolutionary views they have two choices 1. Only admit those that subscribe to the revolutionary programme, and therefore remain small (and therefore ineffective) or 2. Admit all workers and see the revolutionary programme permanently put into the background.

I don't think anything has changed which makes texts like this one wrong:
https://www.marxists.org/archive/malatesta/1925/04/syndic1.htm

Yean, I've recently got involved with a syndicalist union as the IWW are organising in my industry. It is the most exciting thing I've seen in ages and is definitely advancing the class struggle. But having said that I can say from the inside that criticisms of syndicalism from Malatesta onwards still apply.

Yeah, I certainly agree with Darren P's first two sentences - for instance, as much as you and I might disagree with them, supporters of electoralism will say, and I'm sure sincerely believe, that taking part in electoral campaigns helps to organise the working class, as per Eric Blanc and so on.
I suppose my honest answer would be "dunno, your guess is as good as mine really", we haven't got to socialism yet so it's a bit premature to give fully confident answers about what the route is. In general I tend to be skeptical of claims that any one thing is the one correct route. Not to make everything about Spain, but if you look at the heyday of the CNT, you certainly had (lots of) people practicing syndicalism, and people organised in anarchist political organisations with a specific programme like the FAI and Friends of Durruti, and people organised in small informal grupos de afinidad, all of which you can emphasise or de-emphasise depending on what argument you want to make.
As someone who is fairly sympathetic to anarcho-syndicalism, I would say that the bit about "It provides us with a method for putting our principles into action" is an important one, I'm always enthusiastic about people actually connecting theory and practice and it seems to me that some of the best efforts in that direction in recent years have come from parts of the IWW. But then the Angry Workers lot are also inspiring to me, and they'd say they're doing something different, although it has a lot of common ground with syndicalism. And the UVW/IWGB/CAIWU unions again have things in common with syndicalism but I don't think any of them even claim to be syndicalist themselves. So I suppose I think syndicalist principles are useful and important but I wouldn't claim them to be the definitive answer.

The corollary is - are conventional unions, once described as pure and simple, anti-working class?

There is an article from Turkey down below expressing scepticism about the value of trade unions.

In India and elsewhere, unions are often political party affiliates, giving rise to several different organisations, but not as in the case that once existed with craft unions that led to the amalgamation and industrial union movement but based upon political partisanship, not unity.

And shouldn't we be wary of the David North WSWS anti-union stance, contemptuous of any form of compromise contract with employers as a point of principle?

Unions depending on conditions and circumstances will express themselves in a variety of forms that are thought the most suited for the situation and therefore they change and evolve.

When the 1930s brought the CIO strikes, didn't that supersede the need for the IWW?

Do union bureaucracies exist. Undoubtedably. Are they challenged? Frequently.

What was the shop stewards movement of the 19teens and then in the 1960s but means of bypassing union officialdom?

My own experience in the CWU was that our weapon of choice was the wildcat walk-out and sympathy strike. Blacking scab mail. But we also required the power of the solidarity of the whole union to avoid victimisation and selective sackings or a lock-out

Others well-versed in labour history can vouch for the versatility of worker resistance.

But is there also conservatism? That, too, cannot be so easily and casually dismissed.

But an enduring body built to engage in a war of attrition, adapting tactics and methods, is more effective than forever re-inventing the wheel and constantly re-starting from scratch.

The class war is always going to present regardless of the degree and extent of the class consciousness or political maturity of workers. And the intensity and ferocity of that struggle will keep ebbing and flowing

Unions are the protective arm that the working class instinctively lifts up to fend off blows to itself.

In general I definitely hold revolutionary unionism both within and outside the establishment unions to be a very valuable strategic component to achieving communism. Working class self-organization will always be important to revolutionary politics and it's important imho to unite that as formally yet organically as possible.

The mass assembly of workers within unions was simply a reflection of the mass assembly of workers in the companies or sectors within which those unions emerged.

In the advanced capitalist countries, unions (and the labour movement in general) will never recover to their 20c levels, since a strong class identity is no longer confirmed in capitalist production, as it was back when heavy industry assembled masses of proles under one roof.