This topic is the reason that why saṅgha at 1st saṅgayanā arrange tipitaka content like that. Because they realize to keep lifespan of buddhism. See ruben2020's answer above. For pali source. In my opinion, I think this is sāriputta-aggasāvaka's teaching because he authored saṅgītisutta, and abhidhamma. Also this comment appear in ānanda's, and sāriputta's commentary, too.
Vinaya pitaka, cuḷavagga, bhikkhunikhandhaka:
‘‘sace kho tvaṃ, gotami, ime aṭṭha garudhamme paṭiggaṇheyyāsi, sāva te
bhavissati upasampadā’’tiฯ
‘‘seyyathāpi, bhante ānanda, itthī vā puriso vā daharo, yuvā,
maṇḍanakajātiko sīsaṃnahāto uppalamālaṃ vā vassikamālaṃ vā
atimuttakamālaṃ [adhimattakamālaṃ (syā.)] vā labhitvā ubhohi hatthehi
paṭiggahetvā uttamaṅge sirasmiṃ patiṭṭhāpeyya; evameva kho ahaṃ,
bhante, ānanda ime aṭṭha garudhamme paṭiggaṇhāmi yāvajīvaṃ
anatikkamanīye’’tiฯ
atha kho āyasmā ānando yena bhagavā tenupasaṅkami, upasaṅkamitvā
bhagavantaṃ abhivādetvā ekamantaṃ nisīdiฯ ekamantaṃ nisinno kho āyasmā
ānando bhagavantaṃ etadavoca – ‘‘paṭiggahitā, bhante, mahāpajāpatiyā
gotamiyā aṭṭha garudhammā, upasampannā bhagavato mātucchā’’tiฯ
‘‘sace, ānanda, nālabhissa mātugāmo tathāgatappavedite dhammavinaye
agārasmā anagāriyaṃ pabbajjaṃ, ciraṭṭhitikaṃ, ānanda, brahmacariyaṃ
abhavissa, vassasahassaṃ saddhammo tiṭṭheyyaฯ yato ca kho, ānanda,
mātugāmo tathāgatappavedite dhammavinaye agārasmā anagāriyaṃ
pabbajito, na dāni, ānanda, brahmacariyaṃ ciraṭṭhitikaṃ bhavissatiฯ
pañceva dāni, ānanda, vassasatāni saddhammo ṭhassatiฯ
‘‘seyyathāpi, ānanda, yāni kānici kulāni bahutthikāni [bahuitthikāni
(sī. syā.)] appapurisakāni, tāni suppadhaṃsiyāni honti corehi
kumbhathenakehi; evameva kho, ānanda, yasmiṃ dhammavinaye labhati
mātugāmo agārasmā anagāriyaṃ pabbajjaṃ, na taṃ brahmacariyaṃ
ciraṭṭhitikaṃ hotiฯ
‘‘seyyathāpi , ānanda, sampanne sālikkhette setaṭṭikā nāma rogajāti
nipatati, evaṃ taṃ sālikkhettaṃ na ciraṭṭhitikaṃ hoti; evameva kho,
ānanda, yasmiṃ dhammavinaye labhati mātugāmo agārasmā anagāriyaṃ
pabbajjaṃ, na taṃ brahmacariyaṃ ciraṭṭhitikaṃ hotiฯ
‘‘seyyathāpi, ānanda, sampanne ucchukkhette mañjiṭṭhikā [mañjeṭṭhikā
(sī. syā.)] nāma rogajāti nipatati, evaṃ taṃ ucchukkhettaṃ na
ciraṭṭhitikaṃ hoti; evameva kho, ānanda, yasmiṃ dhammavinaye labhati
mātugāmo agārasmā anagāriyaṃ pabbajjaṃ, na taṃ brahmacariyaṃ
ciraṭṭhitikaṃ hotiฯ
‘‘seyyathāpi, ānanda, puriso mahato taḷākassa paṭikacceva āḷiṃ
bandheyya yāvadeva udakassa anatikkamanāya; evameva kho, ānanda, mayā
paṭikacceva bhikkhunīnaṃ aṭṭha garudhammā paññattā yāvajīvaṃ
anatikkamanīyā’’tiฯ
But upāli-etadagga commented, at 1st saṅgāyanā, about above context:
[403] kumbhathenakehīti kumbhe dīpaṃ jāletvā tena ālokena paraghare
bhaṇḍaṃ vicinitvā thenakacorehiฯ
setaṭṭhikā nāma rogajātīti eko pāṇako nāḷimajjhagataṃ kaṇḍaṃ vijjhati,
yena viddhattā nikkhantampi sālisīsaṃ khīraṃ gahetuṃ na sakkotiฯ
mañjiṭṭhikā nāma rogajātīti ucchūnaṃ antorattabhāvoฯ mahato taḷākassa
paṭikacceva āḷinti iminā pana etamatthaṃ dasseti – yathā mahato
taḷākassa āḷiyā abaddhāyapi kiñci udakaṃ tiṭṭheyya, paṭhamameva
baddhāya pana yaṃ abaddhapaccayā na tiṭṭheyya, tampi tiṭṭheyya;
evameva ye ime anuppanne vatthusmiṃ paṭikacceva avītikkamanatthāya
garudhammā paññattāฯ tesu apaññattesupi mātugāmassa pabbajitattā
pañceva vassasatāni saddhammo tiṭṭheyyaฯ paṭikacceva paññattattā pana
aparānipi pañcavassasatāni ṭhassatīti evaṃ paṭhamaṃ vuttaṃ
vassasahassameva ṭhassatītiฯ vassasahassanti cetaṃ
paṭisambhidāpabhedappattakhīṇāsavavaseneva vuttaṃฯ tato pana uttarimpi
sukkhavipassakakhīṇāsavavasena vassasahassaṃ, anāgāmivasena
vassasahassaṃ, sakadāgāmivasena vassasahassaṃ, sotāpannavasena
vassasahassanti evaṃ pañcavassasahassāni paṭivedhasaddhammo ṭhassatiฯ
pariyattidhammopi tāniyevaฯ