It is often said, that verbs in English inflect to agree with the person and number of the subject. Now person here cannot be construed as an actual property of the subject. We cannot say for example that the first person is the person who's speaking and the second person is the person being spoken to or any ideas like that. If we do not use an actual pronoun to represent the person speaking, then the verb will not inflect in any way to agree with the 'speaker-hood' of the subject.
- *Araucaria am writing this. (wrong)
- *The reader are reading this. (wrong)
In the sentences above, even though Araucaria refers to the person who is currently speaking to you, in other words me, we cannot use a 'first person' form of the verb. Similarly, even though the reader refers to you, the 'listener', we cannot use a second person form of the verb. When we use common nouns we do not see agreement for person.
Although with common nouns, and proper nouns English verbs inflect to agree with number, they do not agree with any other property of the noun, including the relation to speaker or listener or third party. However, verbs do seem to inflect according to which pronoun is being used. In other words pronouns override the normal agreement of verbs with subjects:
- I am writing this
- You are reading this.
The examples above are fine, not because I refers to the person who is speaking, and you are the person reading, but because verbs inflect in accordance with specific pronouns, and these pronouns override the normal agreement that we see with common nouns.
In the Original Poster's question, this issue is disguised, because when verbs agree with third person singular and plural pronouns, they mimic their behaviour with common nouns. However, this is just an illusion. As with the pronouns I, you and we, 'they' also overrides the normal agreement of verbs with common nouns. Whether used with singular or plural meaning, pronouns always dictate the agreement of the verb according to which actual pronoun they are. They always takes a 'third person plural' form of the verb.
This same phenomenon can be seen with the pronoun one. Whether used to reflect first person, second person, or people in general, one always takes the same verb agreement, the one we wrongly describe as 'third person singular'. The same also applies to royal we.
The answer to the Original Poster's question, therefore, is that verbs won't inflect to reflect the singularity of singular they, because although verbs agree with number when they have common noun subjects, using a pronoun as subject will override the normal common noun agreement and cause the verb to agree according to which specific pronoun is being used. It wouldn't be a good idea to use 'third person singular' forms with they, because it would just be ungrammatical. It wouldn't reflect anything about the meaning of they. The agreement of English verbs with pronouns never reflects any semantic property of the pronoun in the first place!
... is definitely ungrammatical. We most definitely need:
The reason for this is that verbs agree with the pronoun they, not the amount of people or things it refers to!
[Readers who are interested in this question might also be interested in: Why don't we say "I is" instead of "I am"? - Pronoun usage and conjugation - although it is a slightly strange question!]
This is basically a duplicate of an answer I wrote here