Scab has no connection with something being temporary. Its association is with morality.
Scab was originally a disease of some sort. In such a context, it was never a positive thing:
1.a. Disease of the skin in which pustules or scales are formed: a general term for skin diseases, but sometimes spec. = itch or scabies
(also, dry scab), ringworm or tinea, syphilis; wet scab, eczema.
c1250 Si lepre [signefieþ] þo sennen, þet scab bi-tokned þo litle
sennen. Kent. Serm. in Old English Miscellany 31
With less negative force it then developed the current medical meaning of
The crust which forms over a wound or sore during cicatrization.
c1400 Anoynte al his heed..til al þe scabbis þerof be wel tobroke.
It then developed a figurative meaning (Not unreasonably as disease and other infirmities were said to be punishment for offending God.)
1.b.† figurative. Applied to moral or spiritual disease.
?1529 This is the great scabbe why they will not let the newe testament go a brode yn your moder tong. S. Fish, Supplicacyon for Beggers.
At, or close to, the same time, we have the application to people of dubious moral character - as found in the idea of a strike-breaker:
4. slang. A term of abuse or depreciation applied to persons:
4.a. A mean, low, ‘scurvy’ fellow; a rascal, scoundrel. †occasionally applied to a woman.
a1592 Loue is such a proud scab, that he will neuer meddle with
fooles nor children. R. Greene, Frier Bacon (1594)
This lasted until at least the late 19th / early 20th century
1899 You're three beastly scabs! R. Kipling, Stalky & Co.
The meaning in question first appears in the late 18th century and combines the moral and spiritual aspect found in the 16th century and applies it to the OP’s example
4.b. A worker who refuses to join a trade union, guild of fellow workers, etc.; in extended uses: a person who refuses to join a strike
or who takes over the work of a striker; a blackleg; a
1777 To the Public. Whereas the Master Cordwainers have gloried, that
there has been a Demur amongst the Men's and Women's Men;—we have the
Pleasure to inform them, that Matters are amicably settled... The
Conflict would not been [sic] so sharp had not there been so many
dirty Scabs; no Doubt but timely Notice will be taken of them. Bonner
& Middleton's Bristol Journal 5 July
1792 What is a scab? He is to his trade what a traitor is to his country... He first sells the journeymen, and is himself afterwards sold in his turn by the masters, till at last he is despised by both and deserted by all. in A. Aspinall, Early Eng. Trade Unions (1949)