The study of the compressibility of repetitive sequences is an issue that is attracting great interest. We consider purely morphic words, which are highly repetitive sequences generated by iterating a morphism$\varphi$that admits a fixed point (denoted by$\varphi^{\infty}(a)$) starting from a given character$a$belonging to the finite alphabet$A$, i.e.$\varphi^{\infty}(a)=\lim\nolimits_{i\rightarrow\infty}\varphi^{i}(a)$. Such morphisms are called prolongable on$a$. Here we focus on the compressibility via the Burrows-Wheeler Transform ($BWT$) of infinite families of finite sequences generated by morphisms. In particular, denoted by$r(w)$the number of equal-letter runs of a word$w$, we provide new upper bounds on$r(\mathsf{bwt} (\varphi^{i}(a)))$, i.e. the number of equal-letter runs produced when$BWT$is applied on$\varphi^{i}(a)$. Such bounds depend on the factor complexity$f_{x}(n)$of the infinite word$x=\varphi^{\infty}(a)$, that counts, for each$n\geq 0$, the number of distinct factors of$x$having length$n$.