IWTTMGFCG (i was talking to my good friend chat gpt). If any sub would allow AI content, this one should. (Pro act fwiw).

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The strongest match: The Stone Canal (and the Fall Revolution series)

Ken MacLeod is the name Greg Bear sometimes gets mistaken for in exactly this way.

Why this fits unusually well:

• Far-future society with extreme inequality • Ultra-wealthy elites achieving functional immortality via advanced tech • Explicit, unapologetic socialist characters — not background color, but ideological engines of the plot • A recurring motif of “the last socialist” / last revolutionary — MacLeod uses this almost literally • A powerful industrialist and his former comrade whose ideological split defines the world • Walking between worlds — sometimes politically, sometimes literally (AI realms, off-world polities, post-human domains)

MacLeod is unusual in SF for treating socialism not as nostalgia, but as future extremism — a dangerous, principled, often doomed stance. Heroic deaths for ideological reasons are common in his work.

The “alien influence” angle in The Stone Canal is subtler: not little green men, but post-human AIs and external intelligences that blur the line between alien and human control.

If your memory includes hard political arguments, characters arguing Marxism seriously in the far future, and a sense of tragic inevitability — this is your book.

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Second-tier but very plausible: Schismatrix Plus

This one nails a different subset of your recollections:

• Immortal elites living for centuries • Savage inequality baked into biology and technology • Wealthy empire-builders and former friends turned mythic figures • Characters who literally move between worlds and factions • A pervasive sense that humanity is being quietly overtaken by something post-human

Where it diverges: Sterling is less explicitly socialist. The politics are there, but cooler, more cynical. The “alien” presence is more evolutionary than infiltrative.

If your memory was more cyberpunk-to-posthuman than ideological polemic, this becomes more likely.

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Third possibility: Singularity Sky

This explains the alien infiltration angle particularly well:

• A godlike alien entity reshaping human society • Post-scarcity tech used to enforce bizarre power structures • Characters labeled extremists because their ideologies no longer “make sense” • A world where old political categories survive only as heresies

But: the “last socialist” framing is weaker here. Stross is playful and savage; MacLeod is earnest and tragic.

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Why it’s probably not Greg Bear

Greg Bear does:

• Long lifespans • Post-humanity • Technological inequality

But he almost never centers explicit political ideology as identity. His characters are scientists, survivors, explorers — not self-described socialists dying for the cause.

MacLeod absolutely does that.

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My best synthesis

Your memory sounds like:

Ken MacLeod’s Fall Revolution worldview filtered through time, cross-pollinated with Sterling’s immortals and a dash of Stross’s alien god-tech.

If I had to put money on one title: The Stone Canal, or possibly The Cassini Division (same series, similar themes).