You are right to question this, and your point about the period after Jambuswami is important.

Let’s separate the issues clearly and keep only the traditional Jain position.

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1. What is kṣāyik samyaktva?

• Kṣāyik samyaktva = right faith (samyak-darshan) that has arisen by total destruction (kṣaya) of darśan-mohanīya and anantānubandhī mohanīya karmas.
• Once attained, it never gets lost; it continues even after moksha (the Siddha also has this samyaktva).

So, in essence, kṣāyik samyaktva is the final, irreversible form of samyaktva.

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2. Connection with Keval-jñān and time-cycle

Traditional doctrine (both Digambar and Śvetāmbara) accepts:

• In this current avasarpinī (descending half-cycle), in Bhārat kṣetra,

- there will be only 24 Tīrthaṅkaras, - and after Bhagwān Mahāvīr, there were only three anubaddha kevalīs: Gautam Swāmī, Sudharmā Swāmī, Jambuswāmī.

• Jambuswāmī is the last kevalī of this avasarpinī in Bhārat kṣetra.

Digambar karmagranthas and later ācāryas are very explicit that:

• Kṣāyik samyaktva can only first arise in Jin-kāl, i.e. the era in which Tīrthaṅkaras/kevalīs are possible (3rd and 4th ārā in Bhārat kṣetra).
• We are now in the 5th ārā, a time when no new keval-jñān can arise.

From this, ācāryas conclude:

• After Jambuswāmī’s nirvāṇ, in this avasarpinī and in this region,

no new soul will attain kṣāyik samyaktva or keval-jñān.

• The only souls who “have” kṣāyik samyaktva now are the Siddhas who already attained liberation.

So the current attainable forms of samyaktva in our era are kṣayopaśamik (mixed destruction–suppression) and aupaśamik (pure suppression), not fresh kṣāyik samyaktva.

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3. “Did Bhagwān Mahāvīr say it will be extinct after Jambuswāmī?”

In the way it’s usually quoted in talks – “Mahāvīr himself said: after Jambu there will be no kṣāyik samyaktva” – that exact sentence is not preserved as a commonly cited, word-for-word āgamik sūtra.

What we do have is:

• Mahāvīr’s omniscient teaching about the time-cycle (which includes the limit of kevalīs in this avasarpinī and the description of Jin-kāl).
• Later āgamas and karmagranthas, and the explanations of ācāryas, which derive from those teachings that:

- Jambuswāmī is the last kevalī, - and kṣāyik samyaktva can arise only in the eras when kevalīs/Tīrthaṅkaras appear. Therefore, after Jambu’s nirvāṇ in this time-cycle and region, no new kṣāyik samyaktva arises.

So the doctrinal conclusion you are pointing to is correct: for us, in the present era, kṣāyik samyaktva is not something that will newly arise.

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4. Sect-wise note

• Both Digambar and Śvetāmbara:

- Accept Jambuswāmī as last kevalī of this avasarpinī in Bhārat kṣetra.

• Digambar texts are more explicit in stating:

- kṣāyik samyaktva first arises only in Jin-kāl, - which does not include our present 5th ārā.

• Śvetāmbara ācāryas also accept that we are in a non-liberating era (no new kevalīs), so in practice the result is the same: we do not talk about people today newly attaining kṣāyik samyaktva.

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5. Correcting the earlier impression

So, to be completely clear and aligned with traditional Jain siddhānta:

• Today, in our era and region,

- a soul can attain samyaktva in the forms of kṣayopaśamik or aupaśamik, - but cannot newly attain kṣāyik samyaktva or keval-jñān.

• Kṣāyik samyaktva exists eternally in Siddhas, but no fresh kṣāyik samyaktva will arise after Jambuswāmī in this avasarpinī in Bhārat kṣetra.

On that substantive point, your objection is valid, and my earlier explanation (if it suggested that kṣāyik samyaktva is still attainable now in the same way) needs to be revised exactly along the traditional lines described above.

For a simple overview of samyaktva in general (not specifically this time-cycle detail), you can also see this explanation on JainKnowledge: