The speaker laments that his muse is 'decayed' now that the beloved is praised by another, better poet, and philosophizes that all praise ultimately returns to the beloved's own virtue.
Sonnet 79 performs an intellectual reversal: the rival poet is not a threat but a thief who steals and returns, creating a circular economy. He 'robs thee of, and pays it thee again'—taking the beloved's virtue and fashioning it into praise. This is elegant philosophy but also evasion. By insisting the rival only recirculates what already exists in the beloved, the speaker diminishes the rival's creativity while also diminishing his own. If all praise is merely reflection of intrinsic beauty, then the speaker's own verses are equally derivative. The consolation that 'no praise to thee, but what in thee doth live' might comfort the beloved but leaves the speaker displaced. He has become irrelevant to a process that is purely between the beloved and their own beauty.
Line 3-4 are devastating: 'But now my gracious numbers are decayed, / And my sick muse doth give an other place.' The speaker's verse has literally sickened because the muse has been divided. This reflects a psychological crisis: if the beloved's inspiration can flow to multiple poets, it is not the speaker's alone. The beloved is not 'possession' but a universal good, available to anyone with the eloquence to ask. The speaker's previous confidence (78) that he alone truly benefits from the beloved's presence collapses. The muse does not belong to him. The beloved's attention is divisible. And this knowledge has made his verse sick—not because he lacks talent but because the foundation of his inspiration (exclusive access, singular devotion) has been undermined.
An artist watches someone else's work go viral with their muse as subject. They think: 'Well, technically everything they say about you is yours already. They're just reflecting what you already are. But that doesn't help me. I've lost my exclusive access to your inspiration.' The philosophy rings true but doesn't ease the wound.