In what sense is the love of the two people in "Sonnet 75" still alive today?

The love described in "Sonnet 75" is undoubtedly extant; while it may not be that love of a poet for his patron as is conveyed in this particular sonnet, the expression of love that is written can also apply in modern times to that between close friends or between lovers.
In this particular sonnet, the poet addresses his sponsor, the Earl of Southhampton. While this love for the young man hints at erotic expression, it also conveys feelings shared in any loving relationship. Perhaps the lines that best express these emotions are the following:

And for the peace of you I hold such strifeAs 'twixt a miser and his wealth is found;Now proud as an enjoyer, and anonDoubting the filching age will steal his treasure

These lines connect to those who are in love during any era. While reveling in the warm and happy sensations of love and cherishing the loved one, there is at the same time a fear of losing this love, just as the miser delights in his gold but then worries that someone will steal it from him. Indeed, these feelings remain in existence in the hearts of those who are in love.