It's quite apparent that you've completely missed the overarching point of this discussion.

> It matters to your statement, which is wrong as you made it.

What part exactly? You speak from narrow anecdote in your limited encounter with 1, maybe 2, online service providers and have provided precisely zero supporting reference otherwise. I've provided corroborating citation direct from the IRS.

> It matters whether the "code" needed is...and can thus easily retrieve.

No, it really doesn't. All that matters is that it's easily misplaceable or forgotten...the more arbitrary and useless, the better it'll serve the objective role of vendor lock-in in a world where options abound and financial incentives to the filer are the same as a direct consequence of the inherent simplicity of their case filing.

You've also made certain unjustified presumptions on the ease of retrievability. Once upon a time, at least one well known service provider would produce a digital summary of your filing only after the IRS had accepted the return (not to be confused with receiving it), and this personal copy was generated for free if and only if it was downloaded before the tax year rolled over. So if you forget to download it for personal record, they'd gladly reproduce the summary any time in the future...for a nominal fee. In the past, this dark pattern disincentivized e-filing with a different service provider in subsequent years, and it wouldn't surprise me if some still pull this tactic.

> Everyone should be retaining...

What everyone should be doing from a records management perspective is both completely irrelevant to this discussion and disconnected from happenings in the real world, especially as it pertains to individuals on the lowest rung of the socioeconomic ladder. It might surprise you at how many people actually fail to maintain a copy of their vehicle's registration and proof of insurance in the only vehicle that it would be applicable to, or how many people don't even have a copy of their own birth certificate. These are pretty damn important documents, yet surprise: the human condition is real and people are inherently lazy as fuck. That you actually expect the poorest and least educated in society in general to maintain annual records on something as obtuse as tax returns is quite naive. Do you really think the tax return service industry isn't actively exploiting the crap out of basic psychological shortcomings of society to the benefit of their bottom line??

> More effective mechanism for what?

It's the whole point of my original remark: identifying the fundamental mechanisms which are largely responsible for producing repeat conversion in an industry where options abound, the product is free, and little to no differentiating factors of value exist between competing service providers.

At the ass end of the totem pole, the game of repeat conversion isn't about which service provider is going to somehow provide a filer with the biggest tax return as the parent asserted, and to which I disagreed...we're talking about candidates who qualify for free filing; these are the simplest of turnkey cases, hence why the service is offered for free to begin with! It doesn't matter what you think about the pragmatic utility of the self-service PIN because the fact is it's one of two options mandated by the IRS as a requirement to e-file. Some service providers will require you to produce last year's self-service PIN to e-file, some will require last year's AGI...wouldn't surprise me if some require both.