Context. Massive stars play a very important role in many astrophysical fields. Despite their scarcity with respect to less-massive counterparts, their influence on the chemo-dynamical evolution of the galaxies is substantial. Yet, some fundamental aspects of their evolution remain poorly constrained. In this regard, there is an open debate on the width of the main-sequence (MS) phase, in which stars spend most of their lifetimes. Aims. We aim to create an updated Hertzsprung-Russell (HR) diagram that includes a volume-limited and statistically significant sample of massive stars with luminosities L ≳ 2 × 104 L⊙ and effective temperatures Teff ≳ 14 kK. Our goal is to use this sample to investigate the extension and end of the MS while also incorporating information about projected rotational velocities (v sin i) and spectroscopic binarity status. Methods. We combined spectroscopic parameters derived with the FASTWIND stellar atmosphere code and Gaia distances to obtain estimates of stellar parameters for a sample of 876 Galactic luminous O- and B-type stars gathered within the IACOB project. We used the ALS III catalog of Galactic OB stars to select the best-suited volume-limited sample for our study. We chose the iacob-broad tool to derive v sin i estimates and reviewed multi-epoch spectra to identify single- and double-line spectroscopic binaries (SB1, SB2). Results. We present an HR diagram for a sample of 670 stars located within 2500 pc that has the best balance between completeness and number. We evaluated the extension of the MS in terms of the drop in the relative number of stars as a function of the effective temperature for different luminosity ranges. We found a consistent cool boundary at ≈22.5 kK within the full range of luminosities that we used to delineate the terminal-age main sequence (TAMS). We obtained a smooth decrease of the highest observed v sin i with Teff along the MS band, which is likely limited by the critical velocity. We consider this effect combined with a lower expected fraction of stars beyond the MS as the best explanation for the lack of fast-rotating objects in the post-MS region. Our results favor low to mild initial rotation (vini ≲ 150 km s‑1) for the full sample along with a binary past for the well-known tail of fast-rotating stars in the v sin i distribution. The prominence of SB1 and SB2 systems within the MS band and the 25% decrease in the relative fraction of SB1 systems when crossing the TAMS can further delineate its location.