This is wrong, and let me explain you why. First of all, i will post some calcs against Pokemon that Porygon2 could wall/check but not OHKO without Protean, so it ended up taking too much damage in the process (will be using a max HP / max SpA+ spread):
- 252+ SpA Protean Porygon2 Thunderbolt vs. 0 HP / 4 SpD Manaphy: 278-330 (81.5 - 96.7%) -- 62.5% chance to OHKO after Stealth Rock
- 252+ SpA Protean Porygon2 Thunderbolt vs. 252 HP / 4 SpD Azumarill: 336-396 (83.1 - 98%) -- 68.8% chance to OHKO after Stealth Rock
- 252+ SpA Protean Porygon2 Thunderbolt vs. 4 HP / 0 SpD Mega Gyarados: 222-264 (66.8 - 79.5%) -- 31.3% chance to OHKO after Stealth Rock
- 252+ SpA Protean Porygon2 Ice Beam vs. 4 HP / 0 SpD Latios: 258-306 (85.4 - 101.3%) -- 81.3% chance to OHKO after Stealth Rock
- 252+ SpA Protean Porygon2 Hidden Power Fire vs. 252 HP / 4 SpD Mega Scizor: 372-444 (108.1 - 129%) -- guaranteed OHKO
- 252+ SpA Protean Porygon2 Shadow Ball vs. 4 HP / 4 SpD Aegislash-Shield: 174-206 (66.4 - 78.6%), not a OHKO but a pretty impressive number
- 252+ SpA Protean Porygon2 Psychic vs. 0 HP / 252 SpD Assault Vest Conkeldurr: 192-228 (54.7 - 64.9%) -- guaranteed 2HKO (what would be considered as a perfect counter before)
There are more OHKOs and 2HKOs, and there are other spreads as well, but this should do for what i want to prove.
Furthermore, there is always the potential to use Protean defensively. As i already mentioned, some of the best checks to Porygon2 are Fighting-types, and by carrying Psychic, Toxic, or Shadow Ball you can completely turn the tables in your favor, as long as you hit them on the switch.
Finally, Trick Room + 3 attacks is always a possibility, and with 105 SpA, Life Orb, good bulk, and a wide movepool, Protean Porygon2 could become a good cleaner.